In Our Time – The Mariana Trench

BBC Radio 4 – February 19, 2026
Host: Melvyn Bragg
Guests: Dr. Heather Stewart (Kelpie Geoscience, University of Western Australia), Prof. Alan Jamieson (Deep Sea Research Center, University of Western Australia), Prof. John Copley (University of Southampton)

Main Theme

This episode explores the mysterious and extreme environment of the Mariana Trench—the deepest part of the world’s oceans. The host and panel discuss the trench’s geological structure and formation, the profound challenges of deep-sea exploration, the unique biologic communities surviving in the hadal zone, how life adapts to such extreme pressure, and the human and technological impacts upon these remote places.

Key Discussion Points & Insights

1. What is the Mariana Trench?

[03:00] Heather Stewart

• It's a subduction trench—a long, arc-shaped depression formed where the Pacific Plate is thrust beneath adjoining continental plates, part of the Pacific "Ring of Fire."
• At its deepest—Challenger Deep—it reaches 10,925 meters, about 98 times the height of St. Paul’s Cathedral.
• The Trench arcs roughly 2,550 km around the Mariana Islands.

2. First-Hand Accounts of Deep Dives

[04:31] Heather Stewart & [06:30] Alan Jamieson

• Heather describes the awe and excitement of descending into deep trenches by submersible:
  • “That color change as you start to fall through the water column ... clear waters ... brightest shades of blue, down to absolute pitch blackness ... the silence and the excitement ... your brain processing what you’re seeing out the viewports, trying to put that into geological context.” [04:31]
  • The thrill of being the first human to view that landscape.
• Alan shares an unplanned dive to the Sirena Deep (10,700m) and the surprising efficiency of the mission:
  • “We did one every two days for a week ... Nobody expected us to do that ... So it wasn’t planned, it wasn’t really supposed to happen.” [06:30]

3. Engineering Challenges of Deep-Sea Exploration

[08:14] Alan Jamieson

• The pressure down there is about 1 ton per cm². Modern materials (e.g., titanium spheres) make engineering feasible by simply "making things thicker and thicker."
• Communication, not pressure, is a major challenge:
  • “The biggest problem, is trying to punch an acoustic signal through 7 miles of water. … It’s really bizarre—you can almost hear the accent in it ... We’ve got text messages now as well, which is quite nice.” [09:22]

4. Geology of the Trench

[14:27] Heather Stewart

• The trench operates as a conveyor belt: dense oceanic plate subducts beneath the overriding plate.
• The deep seascape varies:
  • Covered in hemipelagic, clay-rich sediments and volcanic rocks (basalts).
  • Exposed mantle rocks seen as “massive dark rocks ... with a lovely sheen ... major faults and joints.” [15:27]
  • Unique vent systems, e.g., Shinkai vent—“white, pristine chimneys” rather than the typical “black smokers.”

5. Turbidity Currents

[17:24] Heather Stewart

• They’re massive underwater avalanches or “underwater waterfalls,” transporting sediment and nutrients, triggered by gravity, earthquakes, or volcanic eruptions.
• Essential for transporting food/nutrients to trench-dwelling life.

6. Life at the Extreme Depths

[18:31] Alan Jamieson

• Fauna shifts dramatically below 8,000 meters:
  • Above this, fish, prawns, urchins, sea stars, squid; below, mostly small creatures like amphipods, isopods, polychaetes, jellyfish, and anemones.
  • Notably, the anemone "Galatheanthemum" is only found at the absolute deepest points: “You have to be deeper than about 8,500–9,000 meters to see it ... they look like a little white flower ... really quite beautiful looking thing.” [19:45]
• Snailfish discovery:
  • “We named the Mariana [snailfish] because it was quite prestigious ... the deepest fish in the world ... they all look goofy and weird ... family of fish, snailfish, are not actually deep-sea fish—they’re shallow-water fish that have completely taken over.” [20:06]

7. Adaptation to Pressure

[22:29] John Copley

• Bodies filled with incompressible liquid and solid matter (not gas) withstand pressure; mechanical pressure is less of a challenge.
• Real problems: protein folding and membrane fluidity under extreme pressure.
  • Organisms possess “chaperone molecules” to help proteins fold and special lipids to keep membranes flexible.
  • “They’re not mechanically withstanding pressure … for deep sea life there is a challenge, but it’s about what happens with molecules in cells.” [22:29]

8. Dynamic Geology and Instability

[25:55] Heather Stewart

• Very active zone: “Volcanic eruptions ... earthquakes ... it is quite a ‘slippy’ boundary at the Mariana ... constantly evolving and changing.” [25:55]
• Earthquakes can trigger tsunamis and source vast sediment flows.

9. Human Impact at the Deepest Points

[27:37] Alan Jamieson

• Human debris is present in almost every dive:
  • “I don’t necessarily recall any dive that I haven’t seen something man-made ... 19 plastic bags ... even an eco-friendly plastic bag—how eco-friendly is that?” [27:37]
  • “Whole Western side of Challenger Deep ... now a no-go zone, covered in discarded fiber optic cable ... hours and hours of footage of fiber optic cable ... very, very dangerous.” [28:31]
• Military artifacts:
  • “There’s an SR71 Blackbird ... somewhere in the Mariana ... they took it out of Guam and threw it off the back.” [30:46]

10. Food Webs & Sources of Life

[31:17] John Copley

• Main food source: “marine snow”—detritus (dead animals, feces) raining down.
  • “Trenches act like funnels ... bottom of your trench, it’s a toilet and a mortuary. But that’s what things will make a meal of.” [31:26]
• Some animals can even digest wood; others thrive around cold seeps (methane-fueled microbial communities).

11. Why Study Multiple Trenches?

[33:32] Alan Jamieson

• Mariana is unique but not representative:
  • “If you’re trying to understand high-altitude biology ... how much would Everest tell you about every other mountain? Not much.” [33:32]
  • Most trenches are near continental margins and receive more organic input; the Mariana is isolated, oligotrophic, and less dynamic.

12. Human-Caused Changes & Climate Impact

[35:17] John Copley

• Even the deepest trenches will feel effects of global change:
  • “Deep ocean ... will end up with about 10% less oxygen than preindustrial times ... that change is already baked in ... it’s on its way to the deep ocean.” [36:22]

13. Cross-Disciplinary Collaboration

[37:21] Heather Stewart

• Successful expeditions require geologists, engineers, chemists, biologists thinking creatively together:
  • “Each discipline thinks a little bit differently ... that fusion and that sort of spark ... makes it all worthwhile.” [37:21]

14. Studying Deep-Sea Animals

[39:24] John Copley

• They don’t explode when brought up (“nothing in them to expand ...”), but die or change physiologically.
• Modern methods include “preserving animals at the seafloor,” allowing detailed genomics studies.

15. Perpetuation of Myths

[40:48] Alan Jamieson

• Myths of giant monsters (like Megalodon) persist:
  • “It could never, ever support a large animal, especially not that kind of size ... Just all to do with education and how the deep sea is portrayed in the media.” [40:48]

16. Deep-Sea Mining and Conservation

[41:49] John Copley

• Deep-sea vents (not in subduction trenches) face threats from mining.
  • “Mining active deep sea vents would risk extinction of species ... must be protected.” [41:49]

17. Frontiers of Discovery: The Mariana and Beyond

[42:55] Heather Stewart & [43:47] Alan Jamieson

• Mariana draws interest for its record depth, but other trenches are more geologically active and revealing.
• “Mariana is massive ... the volume about the same as the Himalaya ... you could theoretically spend your entire life just working on that thing.” [43:47]

Notable Quotes & Memorable Moments

• “That moment when you turn on the lights ... and you start to see the sea floor coming up underneath you is absolutely fantastic.” – Heather Stewart [04:31]
• “Nobody expected us to do that ... it wasn’t really supposed to happen.” – Alan Jamieson [06:43]
• “The biggest problem, is trying to punch an acoustic signal through 7 miles of water ... weird—you can almost hear the accent in it.” – Alan Jamieson [09:22]
• “They live in a little tube and they look like a little white flower, really quite beautiful looking thing ... We can’t find them anywhere else except at the very deepest points.” – Alan Jamieson on Galatheanthemum anemone [19:45]
• “Mainly, they’re feeding on what rains down into the trench ... so the bottom of your trench, it’s a combination of both a toilet and a mortuary.” – John Copley [31:26]
• “I don’t necessarily recall any dive that I haven’t seen something man-made ... eco-friendly plastic bag ... is that how eco-friendly is that?” – Alan Jamieson [27:37]
• “If you’re trying to understand high altitude biology ... how much would Mount Everest tell you about every mountain? Not very much.” – Alan Jamieson [33:32]
• “It is a very dynamic environment ... constantly evolving and occurring process ... constantly changing.” – Heather Stewart [26:35]

Bonus Segment – Fascinating Extra Details

[44:55–59:44]

• Inner Space Race: The race to the bottom of the trench paralleled the space race, with French and US navies competing in the 1960s.
  • “They wanted to be the people to get to the deepest point for the first time ... mirroring the space story to some extent.” – John Copley [45:48]
• Pacific Ocean as Ancient Remnant:
  • “Pacific is the last remnant of that ancient ocean ... oldest oceanic rocks on our planet.” – Heather Stewart [46:09]
• Sounding for Depth: Early soundings used miles of hemp rope; later, innovations included Lord Kelvin’s piano wire and even “bomb sounding" (timing echoes from dynamite explosions).
  • “There’s a scientific paper with Bob Fischer stood on the back deck with a ... case of TNT just like lighting it off a cigar and just lobbing it into the ocean.” – Alan Jamieson [52:53]
• Snailfish’s double jaw:
  • “They have two mouths ... a second jaw inside its head that ... grinds the animal to make sure it’s dead.” – Alan Jamieson [57:09]
• Biological Inspiration for Science:
  • Deep-sea organisms inspire new materials and medical research—e.g., scaly-foot snail for solar panels; chaperone molecules aiding human protein research [58:42]

Segment Timestamps

• Introduction and context: [01:18–03:00]
• Geology and structure: [03:00–04:22]
• First-person diving experiences: [04:22–07:59]
• Engineering hurdles: [08:14–09:45]
• Discovery and naming history: [09:51–14:14]
• Geology at the bottom: [14:14–17:24]
• Sediment flows / Turbidity currents: [17:24–18:23]
• Deep-sea life and adaptation: [18:23–25:26]
• Geologic instability and hazards: [25:26–27:31]
• Human trash and intrusion: [27:31–31:08]
• Deep sea food web: [31:08–33:32]
• Comparative importance of other trenches: [33:32–36:18]
• Climate impact on the deep sea: [36:18–37:21]
• Multidisciplinary collaboration: [37:21–39:06]
• Methods to study deep-sea animals: [39:06–40:48]
• Popular deep-sea myths: [40:48–41:24]
• Mining and conservation: [41:24–42:37]
• Future research and curiosity: [42:37–44:35]
• Bonus/Extra content: [44:47–59:44]

Tone & Takeaways

The conversation integrates awe and respect for the unknown, excitement about scientific process and discovery, and concern about both the practical limitations of technology and the impact of human activity. The guests' camaraderie adds warmth, while their anecdotes—from finding a packet of cigarettes on the ocean floor to the double-jawed snailfish—highlight the unexpectedness and humor inherent in exploration.

Recommended for listeners:

• Interested in earth science, marine biology, the history of exploration, and how human activity reaches even the planet’s most remote frontiers.
• Wanting insight into how science, technology, and collaborative creativity meet in the quest for knowledge.
• Keen on stories of the ocean’s most mysterious places, and the small but significant ways humanity leaves its mark.