A strange signal from space

Summary

Unexplainable: "A Strange Signal from Space" – Detailed Summary

Introduction

In the March 5, 2025 episode of Unexplainable, hosted by Vox, the team welcomes Julia Longoria as a special guest. Julia, known for her work on acclaimed series like More Perfect, Rabbit Hole, and Radiolab, is set to launch her new series, All About AI, and will soon join the Unexplainable team permanently. The episode adopts a game show format where Julia engages with the hosts to unravel scientific mysteries.

Game Show Format

The episode features the game show segment "Unexplainable or Not," where Julia is presented with three scientific mysteries. Two remain unsolved, while one has recently been deciphered. Her task is to guess which mystery has been solved. The team comprises Meredith Hodna, Bird Pinkerton, and Ann, each bringing unique insights to the mysteries at hand.

Mystery One: Weird Ocean Holes (Presented by Bird Pinkerton)

Bird introduces the first mystery, focusing on enigmatic holes discovered on the ocean floor by the National Oceanic and Atmospheric Administration (NOAA). In 2022, a NOAA ship exploring the Mid Atlantic Ridge at a depth of 2,500 meters (1.5 miles) encountered a series of almost rectangular holes aligned in a straight row, reminiscent of "a row of stitches" (03:50). These formations, also observed in 2004, bear similarity to fossilized rock holes, suggesting a natural, possibly tectonic origin.

Despite theories ranging from gas bubble activity to the actions of unknown burrowing organisms, environmental DNA analyses yielded no matches in existing databases, leaving the phenomenon unexplained (05:31). Julia expresses skepticism about theories like gas bubbles forming such precise shapes, indicating the mystery remains unresolved (07:15).

Mystery Two: Roman Concrete Durability (Presented by Meredith Hodna)

Meredith delves into the second mystery concerning the extraordinary longevity of Roman concrete. Structures like the Pantheon’s unreinforced dome have withstood the test of time and harsh marine environments, puzzling modern scientists. Contemporary concrete typically has a lifespan of 50 to 150 years, whereas Roman concrete has lasted over two millennia (08:35).

The mystery extends beyond mere material composition; it involves understanding the unique chemical processes employed by the Romans. Factors such as the use of specific volcanic ash and unconventional mixing techniques have been hypothesized but not fully explained. Julia humorously remarks on the precariousness of modern infrastructure built on "shaky foundations," highlighting the contrast with ancient Roman engineering (12:55).

Mystery Three: The Wow Signal (Presented by Host)

Inspired by Julia’s experience with Radiolab, the host introduces the third mystery: The Wow Signal. Detected in Ohio in the 1970s by the Big Ear Radio Telescope, the signal was a single, narrow-frequency spike that has never been replicated or fully explained (13:00). Its frequency coincides with hydrogen emissions, a universal constant that made it a candidate for extraterrestrial communication (14:05).

Despite excitement, Dr. Yvette Sendis, a radio astronomer from the University of Oregon, remains unconvinced by the alien hypothesis. She suggests potential terrestrial causes such as satellite interference, though these explanations don't perfectly account for the signal's characteristics (16:32). The mystery persists, leaving Julia uncertain about its resolution (18:06).

Guessing Session and Revelation

Post-commercial break, Julia attempts to guess which mystery has been solved. She opts for The Wow Signal, influenced by her intrigue with space phenomena. However, the reveal comes that Roman concrete is the solved mystery. Despite her incorrect guess, Julia receives an engaging explanation of how Roman concrete's durability was uncovered (23:02).

Solution to Roman Concrete Mystery

Meredith Hodna elucidates the breakthrough discovery by Professor Admir Macšic of MIT. Analysis of ancient Roman concrete revealed millimeter-scale calcium-rich nuggets distributed throughout the material (24:25). Unlike modern homogeneous concrete, these heterogeneous nuggets enable a self-healing process: when cracks form and water infiltrates, the calcium dissolves and recrystallizes, effectively sealing the cracks and preventing further degradation (27:31).

This chemical ingenuity explains the unparalleled resilience of Roman structures and offers a pathway to enhancing modern concrete by introducing self-healing properties, potentially reducing environmental impact and extending infrastructure lifespans (28:07).

Unexplainable Series Trailer: "Good Robot"

In anticipation of her new series, Julia shares a trailer for "Good Robot," which explores the multifaceted implications of artificial intelligence. The trailer features imaginative perspectives from experts, portraying AI as both a potential savior and a harbinger of apocalypse, reflecting the complex emotions surrounding AI advancements (31:35).

Conclusion

The episode wraps up with accolades to Julia for her contributions and a teaser for her upcoming series. The team credits the production crew, acknowledges departing members, and invites listeners to engage with future mysteries and episodes.

Notable Quotes:

• Julia on emotions while tackling mysteries: “I am shaking in my boots. I'm terrified.” (01:37)

• Meredith on the significance of Roman concrete: “Roman concrete is literally healing itself.” (27:44)

• Julia's humorous reflection on modern infrastructure: “I didn't quite realize we were building our societies on like such shaky foundations.” (12:27)

• Host on the Wow Signal’s potential extraterrestrial origin: “I'm convinced that the wow. Signal certainly has the potential of being the first signal from extraterrestrial intelligence.” (15:41)

Key Takeaways:

• Ocean Holes: Mysterious, intentional-looking rectangular holes on the ocean floor remain unexplained, with current hypotheses failing to provide definitive answers.

• Roman Concrete: The ancient Roman technique involved calcium-rich nuggets that enable self-healing, offering insights to revolutionize modern concrete durability and environmental sustainability.

• Wow Signal: An enigmatic radio signal from the 1970s continues to elude conclusive explanations, maintaining its status as a tantalizing prospect for extraterrestrial communication.

Final Thoughts

This episode of Unexplainable seamlessly blends scientific inquiry with engaging storytelling, inviting listeners to ponder some of the most intriguing mysteries of our time. Julia Longoria's participation adds depth and anticipation for her forthcoming contributions to the series.

Transcript

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[Julia Longoria]

I was never really a runner. The way I see running is a gift, especially when you have stage four cancer.

[Ann]

I'm Ann. I'm running the Boston Marathon, presented by bank of America. I run for Dana Farber Cancer Institute.

[Julia Longoria]

To give people like me a chance.

[Ann]

To thrive in life, even with cancer.

[Host]

Join bank of America in helping Ann's cause. Give if you can@b of a.com supportann. What would you like the power to do References to charitable organizations is not an endorsement by bank of America Corporation. Copyright 2025. This is unexplainable or not, the game show where we finally get some answers. This week our guest is Julia Longoria. She's been working on a new unexplainable series, All About AI that's going to be coming out next week. And then after that, she's going to be joining the Unexplainable team permanently.

[Julia Longoria]

Woo.

[Host]

Welcome, Julia.

[Julia Longoria]

Thank you. I'm so happy to be here with you all.

[Host]

We are extremely excited as well. How are you feeling about making your debut on solving some high pressure, potentially unsolved mysteries?

[Julia Longoria]

I am shaking in my boots. I'm terrified.

[Host]

We love to welcome people when they're as nervous as possible. So here's what we're doing. Unexplainable or not is a game show where you have to guess what we know and what we don't. You're gonna hear three scientific mysteries and you're gonna hear them from me. From our supervising producer, Meredith Hodna.

[Meredith Hodna]

Hey, there.

[Host]

And from our senior reporter, producer Bird Pinkerton.

[Ann]

Hello.

[Host]

Two of these mysteries are still unexplainable, but one of them has recently been figured out. After you hear all the mysteries, you're going to get a chance to guess which one you think scientists have actually explained. And this week, in honor of you, we're doing a whole Julia themed show.

[Julia Longoria]

Oh, my goodness.

[Host]

You've worked on some of our favorite shows out there. From More Perfect to Rabbit Hole to Radiolab. It's honestly kind of crazy how many Amazing shows you've worked on, guys.

[Julia Longoria]

Thanks.

[Host]

And each of us is gonna tell you a mystery related to one of those shows.

[Ann]

Ish.

[Julia Longoria]

I am so intrigued by what that means. Like, I just don't know. I'm really excited to hear what the mysteries are.

[Host]

Let's just say don't get your hopes up on the connections. But first up, you're gonna hear a mystery from Bird.

[Ann]

Hello, Julia. Welcome to the show.

[Julia Longoria]

Hi, Bird.

[Ann]

So I love to overachieve. So I have actually hit two of your past jobs.

[Julia Longoria]

Oh, my God.

[Ann]

In that my mystery is set in the Atlantic. Also the name of a publication you worked for.

[Host]

Okay.

[Julia Longoria]

Now getting how tangential we are. I love it. Okay.

[Ann]

And it's also about holes, like your past show rabbit hole. So these are extremely real connections. But mostly this is just an excuse for me to do my favorite thing, which is to acknowledge that the ocean is full of mysteries that we are trying to understand. And so today I want to focus on an ocean riddle that was turned up by NOAA. That's the National oceanic and Atmospheric Administration.

[Julia Longoria]

Two A's.

[Ann]

Yes, the NOAA. So in 2022, a NOAA ship was sent out to explore a bunch of spots in the Atlantic, especially along the Mid Atlantic Ridge. And in one of their dives, they spotted these weird holes on the ocean floor. So this is like 2,500 meters down.

[Julia Longoria]

Wait, how far down?

[Ann]

So this was 2,500 meters down, like 1.5 miles about.

[Julia Longoria]

Okay.

[Ann]

And like, they look almost rectangular. They're a little over a centimeter wide, a few centimeters long, and they appear in a straight line, like a line of holes, one after another, almost like a row of stitches.

[Julia Longoria]

Yeah, it really looks like a kid playing in the sand, like doing, like.

[Host]

Really deep underwater in the sand.

[Julia Longoria]

Yeah.

[Meredith Hodna]

A mile and a half under the Atlantic.

[Ann]

They don't look unintentional, essentially.

[Julia Longoria]

Yeah, they look very intentional. Yeah.

[Ann]

And it's not the first time that holes like these have been spotted. There were others seen back in 2004 also in the Atlantic. And the scientists who wrote a paper describing those holes noted that they look kind of similar to holes that they'd found in fossilized rock. Which suggests. Right. That they're probably not man made if they're also in the fossil record. But then how are they made? I reached out to NOAA and they sent me a statement that said essentially it could be some kind of process happening. So maybe there's some kind of tectonic activity in the area releasing gas bubbles that come up to the Surface and make these holes in some way.

[Julia Longoria]

I feel like gas bubbles wouldn't make that, though.

[Ann]

Yeah, it's like they're so rectangular looking that I was kind of, like, skeptical as well. But they're also considering organisms that might do this. Like, maybe something's moving along the surface and making these. Or they've considered burrowing organisms because there are little mounds of sediments by the holes. So to kind of check all this out, one of the things that they did was to collect some environmental DNA from the site. So that's basically like, DNA fragments that are just floating around. And the idea there is to look at those DNA fragments and compare them, see if they match up with anything in our, like, vast catalog of basically, like, everything that we have seen before, essentially. And in the statement that they sent, this Lab director named Dr. Alan Collins said the little DNA fragments they gathered near these holes did not match anything in their data sets. There's just so much down at the bottom of the ocean that we have not cataloged yet. Right. That we can't always use our catalogs to figure out what's going on. And so that sort of leaves us with this mystery of, what are these holes? Where did they come from? It feels a little absurd to think that you can encounter something like this 20 years ago and then encounter it again in 2022 and still not have clear answers. But maybe it feels so absurd because we do have clear answers, and this is not a testament to how very little we know about the ocean floor. And it is instead a mystery we have solved.

[Host]

So, Julia, what do you think? Do you think we might have solved this one, or do you think this is an unsolved mystery?

[Julia Longoria]

I feel like we would have solved it. That is what my gut is saying.

[Host]

They are just holes.

[Julia Longoria]

They are just holes.

[Meredith Hodna]

How hard could it be?

[Julia Longoria]

Yeah.

[Host]

All right. Just remember the gut feeling when you start getting more mysteries later on.

[Julia Longoria]

Yeah.

[Host]

Next up, Meredith has her own mystery for you.

[Meredith Hodna]

All right, Julia. Hello, Meredith.

[Julia Longoria]

Hello.

[Meredith Hodna]

My mystery today is all about concrete, which, not a direct tie to your resume, but we'll get there. It'll all come around.

[Julia Longoria]

I'm on the edge of my seat.

[Meredith Hodna]

I know. Fascinating stuff. So concrete is the most used building material in the world. We're pumping out billions and billions of tons of it every year for our buildings, our roads, our dams. Like, concrete is basically the bedrock of modern life. But there's a concrete mystery that stretches back thousands of years to the Roman Empire, just like us. You know, Romans, they use their concrete to build their Roads, their aqueducts, their cities, even their marketplaces. You got some experience with marketplace, right, Julia?

[Julia Longoria]

Nice.

[Meredith Hodna]

And there are temples, like The Pantheon, built 2,000 years ago, this huge, unreinforced concrete dome that you can still see today. And that is exactly the point. That concrete is still there in the middle of Rome. And it. It's not just in beautifully preserved historical buildings. You can see this ancient concrete in seawalls, like, submerged in salt water, getting pounded by waves, the harshest conditions. And these walls are still standing thousands of years later, and we don't know why. Some scientists think it has to do with the chemistry of the Romans, like, raw materials. There's this very special volcanic ash that came from, like, one very specific seaside town that Romans shipped across the empire to make their concrete. Other researchers think that the durability of this Roman concrete came down to how they mixed all of their, like, concrete ingredients together. Either way, it's extremely hard to figure out why this concrete lasts so long, because all we have are, like, the surviving remains, like, chemically altered by millennia of wear and tear and, you know, being exposed to the elements. It's also impossible to tell what was, like, purposeful engineering and what was just kind of like a lucky accident in material science. But one thing that we do know is that our modern concrete doesn't stand up nearly as well.

[Julia Longoria]

That was gonna be my question. Like, what's the shelf life on concrete?

[Meredith Hodna]

Well, so I asked a professor of engineering at mit, Admir Mascic, and he told me that modern concrete has a shelf life of 50 to 100 years. Maybe 150 years. Like, if you're stretching it.

[Julia Longoria]

Wait, we outlive concrete?

[Host]

Yeah, or some concrete. Some concrete, depending how long we live.

[Ann]

Right. But this is, like, a real issue with, like, if you look at the Frank Lloyd Wright building, that was supposed to be, like, a waterfall building or something, and they want to preserve it, but it's, like, falling apart because so much of it is concrete.

[Host]

Like, oh, interesting.

[Ann]

It's very tragic.

[Meredith Hodna]

Totally. And it's not just architectural marvels we're talking about here. Our modern concrete infrastructure is continually crumbling. It needs to be replaced and repaired, even as we're going on to build just, like, more and more and more out of our concrete. And this has a huge environmental toll. The production of concrete is responsible for, like, billions of tons of carbon released into the air, accounting for as much as, like, 8 or 9% of just all greenhouse gas emissions just from concrete.

[Julia Longoria]

Whoa, wait.

[Ann]

Why is it so, like, it's just rock? Why Is it so. Or it's. It's just little. It is concrete. Rock. What is concrete? But why does it emit so much CO2?

[Meredith Hodna]

So the process of taking limestone and making it become like this liquid rock of cement that then gets mixed into concrete, you're basically like heating it up so much that you're stripping out carbon dioxide. It's like pound for pound.

[Ann]

Got it.

[Meredith Hodna]

Every pound of cement made releases just about a pound of CO2 into the air.

[Ann]

It's so inefficient.

[Host]

Okay, cool.

[Meredith Hodna]

Yeah. So figuring out the secret of Roman concrete, what makes it so durable, what makes it last so long, could be the key of making our own long lasting concrete, you know, that doesn't need to keep being replaced and dramatically cut the carbon cost of all of our infrastructure.

[Julia Longoria]

You know, you're really blowing my mind because I'm like, how? I didn't quite realize we were building our societies on like such shaky foundations.

[Host]

Yeah, shaky and like super environmentally harmful.

[Julia Longoria]

Yeah, yeah. Like, why are we doing that? Isn't there? I guess. I don't know.

[Meredith Hodna]

Yeah, maybe they figured it out.

[Host]

So what do you think? Do you think we know why Roman concrete was so durable, or do you think that's still an unsolved mystery?

[Julia Longoria]

I'm leaning toward it's an unsolved mystery, but I guess I just found out that we are building on shaky foundations. So maybe we aren't, but I'm like very highly impressionable.

[Host]

Well, this just hold off till the third mystery.

[Julia Longoria]

Yep.

[Host]

And this one is inspired by your time at Radiolab.

[Julia Longoria]

Nice.

[Host]

Specifically the radio part. I wanna tell you about something called the wow. Signal. So in OHIO in the 70s, there was this telescope called Big Ear. It wasn't a normal telescope with lenses and mirrors. It was a radio telescope, which means it was essentially listening for radio waves and it would kind of output the data that it got on these spreadsheets. And an astronomer was going over this data and he noticed this really weird signal. There's no recording of it, but there is a printout of the data which I can show you here.

[Julia Longoria]

Wow.

[Host]

Wow.

[Julia Longoria]

Someone just wrote wow, exclamation point. I assume that's not the signal, but.

[Host]

No, no, that's. That's where the name comes from. The guy basically circ the signal, which was just this huge spike in energy, and he wrote, wow. So, wow, signal. But the next time that patch of sky where the signal came from was analyzed, it was gone and the signal was never recorded again.

[Ann]

Is there a chance that like a bird fell on something? And no one noticed.

[Host]

Well, the thing is, it wasn't just that this signal never happened again. The signal was kind of inherently strange because it was just in one narrow frequency. So essentially, like one note on the keyboard, which means that it probably wasn't a natural signal from space, because if it was any kind of supernova or a pulsar or just something that happens all the time in space, that would have sent all kinds of frequencies. And the frequency it was sent at happens to be the same frequency that hydrogen emits radio waves at, which got people ultra excited, because this is the frequency that a lot of astronomers and scientists think that aliens would probably choose to communicate with us because they would probably try to communicate on the most universal frequency, you know, most common element in the universe. Hydrogen.

[Julia Longoria]

Wow.

[Host]

And there is actually an international agreement that prohibits broadcasting on this frequency because it might interfere with signals coming from space.

[Meredith Hodna]

Hmm.

[Host]

All of which is why the guy who found this signal a couple years ago, he said, quote, I'm convinced that the wow. Signal certainly has the potential of being the first signal from extraterrestrial intelligence. Now, he might be biased.

[Meredith Hodna]

Yeah, we all want to be that guy.

[Host]

We all want to be the guy who discovers aliens. Right. So I called up a different astronomer just to get her sense of things. She's a radio astronomer, so she also kind of specifically looks for radio waves coming from space. She works at the University of Oregon. Her name is Yvette Sendis, and she's not exactly convinced by the aliens theory. I don't know if that's a shocker.

[Julia Longoria]

But, I mean, I'm of the opinion that, like, if you think we're alone out here, you're delusional, but I agree.

[Host]

But Yvette doesn't think this is the evidence that we're not alone.

[Julia Longoria]

Fair.

[Host]

And she told me there's this theory that maybe a blast of radiation could have hit a hydrogen cloud in deep space, which would explain why this signal was just on the one hydrogen frequency. But no one's ever observed a radiation blast big enough to make the wow. Signal. So Yvette isn't into that either. She basically told me that it's easy to get excited about weird signals in radio astronomy, but given how many radio signals we're just always emitting all the time, if you detect something weird, odds are it's from us, like, maybe a glitch on our end or something. And that's basically what she thinks might explain this. She thinks it might have been a satellite or interference or something. But even that explanation isn't perfect, because the wow. Signal wasn't moving relative to the stars in the background like a satellite would. It seemed like it came from somewhere really far away. Which means that after all of this, we're left with a whole series of imperfect explanations. Unless it was the aliens. And aliens, if you're listening, maybe reach back out and let us know that you're listening a little more clearly than.

[Meredith Hodna]

Just who dis new phone.

[Host]

Yeah, maybe be a little more explicit than hydrogen.

[Bird Pinkerton]

I don't know.

[Host]

So that's where we are. What do you think? Do you think we've figured out the wow. Signal?

[Julia Longoria]

I don't know. I'm on the fence about this one.

[Host]

Sounds like you're on the fence about a lot of them.

[Julia Longoria]

Yeah, perfect. I knew I was gonna be bad at this game.

[Host]

So you've got the mystery of birds, weird deep ocean holes. You've got Meredith's Roman concrete being way more durable than we would otherwise expect. And we've got what potentially caused the wow signal. They're all unexplainable. Or at least they all were unexplainable at one point. One of these potential mysteries has recently been figured out, and you'll have a chance to guess after the break.

[Julia Longoria]

Woo woo woo.

[Host]

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[Bird Pinkerton]

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[Ann]

A Big Mac or 10 piece McNuggets.

[Bird Pinkerton]

With spicy nether flame sauce.

[Ann]

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[Host]

At participating McDonald's for a limited time. A Minecraft movie only in theaters.

[Yvette Sendis]

It's nice to meet you.

[Host]

I'm Julia Coolia. It's unexplainable. Or not. Julia, welcome back.

[Julia Longoria]

Happy to be here.

[Host]

We've got three mysteries to choose from and only one of them has been explained. Mystery one, what are all these weird holes at the bottom of the ocean? Mystery two, why was Roman concrete so strong? And mystery three, what's behind the wow signal? So just kind of tell me what you're thinking. Okay, maybe all of all of your anxieties.

[Julia Longoria]

I'm very anxious right now. Just the baseline assumption I have is that the ocean is such a mystery to us. So I have a feeling that maybe we don't have an explanation for that one. And then as far as the concrete, it does feel like one that we would most feasibly have an answer to, but it's the kind of thing we would, like, just postpone figuring out because, like, big concrete, you know, big concrete.

[Ann]

Doesn'T want better concrete because then we.

[Julia Longoria]

Wouldn'T have to fix the concrete all the time. Like that's, you know, that's big money.

[Meredith Hodna]

It's all an elaborate room.

[Julia Longoria]

It's a concrete conspiracy.

[Ann]

It goes all the way up to the top.

[Julia Longoria]

And yeah, the last one, I mean, I'm puzzled by, but I feel like maybe it has a dumb answer, like bird. Really? Like, I don't know if you were trying to sway me or to bamboozle me with that. Maybe a bird went on there.

[Ann]

I am a bamboozler. Famously, that's true.

[Julia Longoria]

I had that feeling, I'm going to guess and probably be wrong, that the last one, the wow. Is the one that we know.

[Ann]

Wow.

[Julia Longoria]

Wow.

[Host]

Wow. Signal, final answer.

[Julia Longoria]

Wow. Signal, final answer.

[Host]

Okay, locking it in. Here's your answer.

[Yvette Sendis]

Instead of degrading, it gets better. That's what is happening with throwing concrete.

[Julia Longoria]

They cracked open big concrete. They found the answer.

[Host]

Big concrete's got a worry now.

[Julia Longoria]

Who's shaking in their boots?

[Ann]

Julia, you're faced. Two seconds into that quote, it just fell. I know.

[Host]

You were just like, no.

[Julia Longoria]

God damn it.

[Host]

So, Julia, why Roman concrete is so durable has been figured out. And Meredith is gonna tell you how this mystery was solved shockingly recently.

[Meredith Hodna]

Yeah. So it turns out that I, Meredith Hodinot, am the Bamboozler.

[Ann]

Bamboozled.

[Julia Longoria]

It's always the ones you don't expect.

[Meredith Hodna]

I know underneath this smiley facade, it's just bamboozled down to the core. So we just heard from the MIT engineering professor I talked to, Admir Mashic, and he just loves concrete.

[Yvette Sendis]

The beauty of concrete is the fact that you have this incredible freedom of forms. You pour concrete and then it solidifies into a rock.

[Meredith Hodna]

He found out a couple years ago why Roman concrete lasts thousands of years. And he says it all has to do with the mix. So little concrete, 101 concrete is a bunch of sand and gravel and rocks all held together by a glue made from processed limestone and clay called cement.

[Host]

Heard of it.

[Meredith Hodna]

And throughout his career, Admir has looked at samples of Roman concrete from across the Mediterranean. Some samples even, like, preserved at Pompeii. And everywhere he looked, there were these little millimeter scale nuggets of mostly calcium.

[Yvette Sendis]

Yeah, that's a great. I never used a little nugget term, but I think it describes it incredibly well. It's gold for us in a sense that these little nuggets are super calcium rich.

[Julia Longoria]

It's like a. Like a Dairy Queen blizzard that's not fully blizzed. It's like, got the big.

[Meredith Hodna]

Exactly. Yeah. And Edmir says it was generally thought that these nuggets happened because the Romans were just not very good at mixing their concrete.

[Yvette Sendis]

The nuggets were considered bad material processing.

[Meredith Hodna]

But were all the Romans bad at mixing all of their concrete? Like, these nuggets are everywhere.

[Yvette Sendis]

You put so much effort in this material design, and then you don't mix it properly every single time. It's like, guys.

[Meredith Hodna]

And that gave Admir and his team an idea.

[Yvette Sendis]

I started to challenge this, that Romans consistently badly mix their concrete.

[Meredith Hodna]

So cement, the glue of concrete, needs water to solidify. It's an important part of the chemistry that turns the calcium of limestone and other chemicals like silicates into such a powerful binder. But Admir says that he's been able to prove that the way the Romans mixed their cement meant that not all of the calcium combined with the water. These chemical reactions of mixing the concrete would get so hot that some of the water evaporated, leaving these calcium nuggets behind. Our modern concrete is much more uniform.

[Julia Longoria]

It's a Milkshake.

[Meredith Hodna]

Exactly.

[Yvette Sendis]

In the modern, you have all homogeneously distributed calcium, very boring, like super mixed in. Whereas ancient Roman concrete is like super heterogeneous.

[Meredith Hodna]

So let's use our concretes to make some walls. Over time, tiny cracks start to form in the concrete and water starts to leak in. In modern concrete, that's it. It's the beginning of the end. The water gets into the concrete in these cracks, it can thin, freeze, and thaw, making those cracks spread even further. The water could get all the way down to the, like, metal rebar that is like the structural support of the concrete structures. And then it starts eroding and rusting and beginning the whole crumbling process. But in ancient Roman concrete, that water leaks in, it hits one of these tiny calcium nuggets, that nugget dissolves, then recrystallizes and seals up the crack.

[Yvette Sendis]

So what makes Roman concrete magical is the chemistry behind healing process that's not taking place in modern concrete.

[Julia Longoria]

Fascinating.

[Meredith Hodna]

Roman concrete is literally healing itself. And Admir and his team aren't suggesting we make concrete the way Romans used to. There have been like a, a few engineering advances in the last couple of thousand years that are worth hanging on to, but this discovery is inspiring ideas for additives to help modern concrete heal itself.

[Host]

Wait, so why wouldn't we just make concrete the way the Romans used to? Like, if their concrete lasted for 2000 years and our concrete lasts for like 100 years, why wouldn't we just.

[Ann]

I also am confused by this.

[Host]

Make all the stuff with Roman concrete.

[Julia Longoria]

Yeah, right.

[Meredith Hodna]

So the Romans, they knew a lot of things, but they did not know how to make a concrete wall that was not like gigantically thick.

[Yvette Sendis]

In ancient Roman structures, there is no reinforcement, no steel. And so they built the structures with huge walls, thick walls, and everything was very, very bulky.

[Meredith Hodna]

And then while they could make these beautiful domes, it's actually really hard to make flat concrete surfaces without that structural support. So the top of the pantheon is just like this open skylight. And Admir was saying, like, yeah, that's probably because they kept falling down because they couldn't figure out how to structurally support it.

[Yvette Sendis]

Maybe they even tried to make it and were not successful and then just left it. I don't know. I should actually look into that.

[Meredith Hodna]

So if we want to keep our, like, thin walled, elegant concrete skyscrapers with all those, like, real nice flat floors, we gotta figure out how to take lessons from the Romans but not do as they do.

[Host]

So, Julia, even though you didn't get the answer right here, you were Stumped by big concrete. We do have a consolation prize for you, which is a song I wrote.

[Julia Longoria]

Incredible.

[Bird Pinkerton]

The Roman Colosseum somehow hasn't collapsed, even though they built it thousands of years in the past.

[Meredith Hodna]

Wow.

[Bird Pinkerton]

The Roman king beyond's got a dome and it's not reinforced. If we had tried that, it would fall right down on the floor. They still aqueducts. They still sea walls. They're still marketplaces and docks. But finally, scientists figured out why. It's because the Romans built them all with concrete and tiny little husk mixed clothes. They built them with concrete. These nuggets that were mostly made of calcium built them with concrete. It wasn't like they sucked at Nick. Sucked them with concrete. It's more like they just crush and build with all roads lead to roam.

[Host]

And some of them will erode but.

[Bird Pinkerton]

You know what they say about the ones that don't. They were probably built with clumpy Roman concrete. Tiny little on his clothes. They built them with concrete. These nuggets that were mostly made of calcium built them with concrete. Wasn't like they sucked at mixing with concrete. It's more like they just crush and build concrete.

[Host]

And the nuggets. And the nuggets.

[Bird Pinkerton]

And the nuggets.

[Meredith Hodna]

The nuggets.

[Julia Longoria]

Nugget really is an excellent word. What a dream.

[Host]

One last thing before we go, Julia. The Unexplainable series you've been working on. Good Robot comes out next week.

[Julia Longoria]

Yeah, it does.

[Host]

So I wonder if you could give us a little preview.

[Julia Longoria]

I actually have a trailer for you.

[Ann]

Ooh.

[Host]

Okay, let's. Let's listen. Suppose in the future there is an artificial intelligence.

[Julia Longoria]

I've been asking some very smart people a question that's been on a lot of our minds. Should we be worried about artificial intelligence? But the answers I got from the greatest minds in AI Surprised me. One guy told a parable of an AI that could cause an apocalypse.

[Host]

Let's give this super intelligent AI a simple Produce paperclips.

[Julia Longoria]

Be a paperclip. Another woman cast AI as an octopus.

[Host]

We posit this octopus to be mischievous as well.

[Julia Longoria]

And yet another story sounded like it was out of the Bible.

[Host]

She seems likely to drown. What should you do?

[Julia Longoria]

Imagining AI As a savior. Like a God?

[Ann]

Kind of. Yeah, like a God.

[Julia Longoria]

And all of these fantastical tales from the greatest minds in AI made me wonder. Maybe even these people don't know what to think. I'm Julia Longoria. Good Robot, a series about AI coming March 12th on unexplainable. Wherever you get podcasts.

[Host]

Oh, Man, I'm so excited for this, Julia, and we're so glad to have you on the team.

[Julia Longoria]

Aw, thanks.

[Host]

That's it for Unexplainable or Not. Thank you to our presenters, Bird Pinkerton.

[Julia Longoria]

That's me, the Bamboozler.

[Host]

And Meredith Hoddinott.

[Meredith Hodna]

That's my name.

[Host]

Both Bamboozlers.

[Meredith Hodna]

You said it.

[Host]

And thank you to our audience for joining us. If you have a mystery or a solved mystery you want us to tell on an upcoming game show, let us know. We read every single email. That's it for Unexplainable or not. Goodbye. This episode was reported by Bird Pinkerton, Meredith Hadnot and me. Noam Hassenfeld. Production and music for me. Editing from Meredith, who also runs the show, production support from Thomas Lu, mixing and sound design from Christian Ayala, and fact checking from Melissa Hirsch and Anouk Dussaud. Thanks so much to Julia Longoria for playing our game this week. Her Unexplainable series, Good Robot launches next week and after that, you'll be hearing a lot more from her going forward, so get excited. Thanks to Lauren Katz for her help with this episode and honestly, all the game shows we've done so far. Thanks to Brian Resnik for co creating the show. And special thanks to both Anouk and Rob Byers, who are moving on to new projects. Anouk has been an incredibly thoughtful researcher and fact checker for us, and Rob has taught us all more about sound and engineering and just listening than I ever knew was possible. If you have thoughts about the show, send us an email. We're@ unexplainableox.com and you can leave us a review or a rating wherever you listen. It really helps us find new listeners. You can also support this show and all of Vox's journalism by joining our membership program today. You can go to vox.commembers to sign up. Unexplainable is part of the Vox Media Podcast network and we'll be back with the first episode of our series, Good Robot next week.

[Bird Pinkerton]

Concrete and tiny little mugs. Mix nuggets that were mostly made of calcium. Was it like they suck at mix? More like they just crush and concrete.

[Host]

And the nuggets and the nuggets and the knock knock nuggets.