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Mexico City is Mexico's most important city. Today the greater metropolitan area is home to 23 million people, over 17% of the country's entire population. Economically, it is the powerhouse, contributing a quarter of Mexico's gdp. All of the biggest companies are based there. The place is also famously built on top of a lake bed, a sinking lakebed. Sinking may be an overall average of 35-50 cm per year. In this video, we look at Mexico City's land subsidence issues and how they came about. Mexico City, as well as its immediate predecessor, Tenochtitlan, sits in the basin of Mexico. It is a high altitude basin, about 2200 meters high on average in enclosed by volcanoes and mountains. Simply speaking, it's a big natural bowl up in the sky. The valley's climate is generally mild and almost arid. The region recorded about 136 droughts between 1415 and 1900. The annual rainy time covers the short months between July and September. Water used to drain out of the south of the valley to the Pacific. But about 600,000 to 1 million years ago, intense volcanic activity created the Sierra de Chichutnodsen mountain range. The range essentially dammed the valley, keeping water from flowing out of the basin. Thus, water from snowmelt and the rains stayed in the basin and over time it created five interconnected lakes. The largest and lowest lined lake of the five is Lake Texcoco, since since water can only exit via evaporation, the lake became quite salty. Now, before we continue, I want to formally apologize in advance for muffing the pronunciation of all the names in this video. Tenochtitlan was originally founded sometime in the 1320s or 1340s. The city was established on a small artificial island in Lake Texcoco by a Nahuat tribe called the Aztecs, or Mexica, if you would like to call them that. We do not know for sure why they chose this place. It is likely because of the existing presence of wealthy city states on the shores of the basin's various lakes. Since those shores were already occupied, they opted for the island. It was more defensible anyway. The city's only connections to the mainland were a series of causeways. And thanks in part to this defensibility, the Aztecs went on to build a large empire that stretched from the Gulf of Mexico to the Pacific Ocean. At its peak, Tenostitlan was 5 square kilometers large and was home for 200,000 people. When you include those living in the rest of the basin, the total population counted about a million. The city was Notable for its extensive water engineering, it was built by laying earth and rocks on top of the original island. And since the whole thing sat just two meters above water level, floods were often devastating to the city and its residents. After a particularly bad one in the 1440s, the Aztecs built the Nazuacuyotl dike, a flood wall that split off the city from the rest of Lake Texcoco. It would hold for some 70 years. Engineering also had to be done to bring drinking water to the large population. Since all the water in the lake was retained. The waters of Lake Texicoco got pretty salty. So residents turned to springs on the city's outskirts and then built aqueducts to transport water. The Najua Koyoto dike also helped by separating incoming freshwater from the salty water in the rest of the lake. The Spaniards arrived in the 1500s, toppled the Aztecs and colonized the country. The reasons for the Aztecs fall are well discussed. Diseases like smallpox devastated the indigenous population. The Spaniards brought superior weaponry, leveraged the Aztecs fighting styles, and struck alliances with their tributes. In the final battle of the conquest, the Spaniards laid siege to Tenochtitlan itself. Deploying a small fleet of sailing ships across Lake Texcoco, armed with cannon, they sank Aztec canoes, destroyed bridges, and cut off the city's supply of food and water. This finally brought them victory in 1521, but at the cost of major damage to the city's water infrastructure. For instance, Cortes destroyed the aforementioned Nazualcoyoto dike so his ships can pass. Cortes lieutenants urged him to move the city to higher ground or an entirely different area. Coyoacan or Tacuba were considered as possible candidates. But Cortes insisted on staying in Tenochtitlan for political, cultural and institutional reasons. As a minority ruling a majority, he saw it as risky to leave the capital city open. Tribes may try to return and reclaim it. They really should have considered moving. The Spaniards either knew nothing of or ignored the area's flood tendencies. Compounding the city's vulnerable location, the Spaniards eliminated its anti flood water engineering. Canals and ditches built to rout flood water were filled with debris to make European style streets. Surrounding trees were cut down to build stuff which worsened soil erosion. Worse yet, the 1520s were an extended dry period where the lakes receded in size. Needing higher water levels to transport building materials to the city, the Spaniards disassembled existing infrastructure that held back the lake's waters. This was a double mistake. The dry period soon ended in the most dramatic way. The city Found itself hit by massive floods at a rate of about one every 25 years. After a 1555 flood, the levee was rebuilt, but it failed to hold back the waters. Combined with the perceived threat of diseases like typhoid and malaria from bad water, the idea of draining the lakes surrounding the city started to emerge sometime in the 1580s. In 1607, another bad flood hit the city, Causing massive damage to temples, monasteries and homes. The newly ensconced Viceroy of New Spain, Luis de Velasco, Began to worry whether this flooding thing might be a permanent occurrence. So he asked for ideas to solve the problem. Several came in. The winning idea came from a Europe born individual named Enrico Martinez, and he suggested a large canal that would drain one of the lakes, Lake Sumpango, as well as the Kautitlan river, the Nochestongo cut. The cut was a massive undertaking that employed the labor of tens of thousands of indigenous people. Martinez worked on the project for a quarter of the century and faced unending criticism. It first existed as a tunnel, but the basin's soft and shifting soils often caused it to collapse, despite reinforcements of wood and masonry. Then, in 1629, it rained for 40 hours straight, and another major flood gathered. Unfortunately, the gates to the cut were closed, and it's not entirely clear why. Some sources say Martinez was ordered to close the gates by the viceroy at the time. Others say that he closed it himself to prevent rock damage. Whatever the reason why, the massive flood instead swept into Mexico City, and the damage was unprecedented. An estimated 30,000 people died, and tens of thousands more fled to the nearby city of Puebla. The floodwaters did not recede for five years, and damage remained noticeable for another five. Martinez was jailed, but was eventually released because he knew more about the problem than anyone else. Unfortunately, he passed away in 1630, his project not yet completed. His final recommendation was to convert the tunnel into an open gorge. This was initially ignored, but finally in 1637, they started cutting down into the ground to build the gorge. This massive project would not be completed until 1789. Mexico started fighting for its independence in the early 1810s. It did eventually split off from Spain, but then suffered under a series of unstable rulers. In its early history, per capita incomes actually fell from 1800 to 1860. The city's and country's population grew rather tepidly. Population growth in Mexico City came via refugees seeking shelter from political turmoil. But shelter was limited, and the existing water and sewage infrastructure was then already over a hundred years old. Said political turmoil had caused it to fall into disrepair. Military measures during times of war made things worse. During the Mexican American War, the United States army marched towards Mexico City. In 1847, in a desperate attempt to defend the city, the Mexican Republic's president, Santa Anna March, breached the dikes to try and create a moat, or at least an unpassable swamp. This failed and in the process inundated many villages in the east. After a tumultuous few decades, the country found stable footing. In the second half of the 1800s, it began to industrialize, with great repercussions for the urban capital. Mexico City was undoubtedly the country's largest and most important city, but its population had changed little since the days of the conquest. Most of Mexico's 8.4 million people lived in secondary cities like the aforementioned Puebla or Guadalajara. Then, in 1859, Mexico passed the Reform Laws, which forced the Catholic Church and other large estates to sell and distribute its massive landholdings. These estates had been inefficiently organized and and poorly run. Their owners basically absent. The Reform Laws allowed the metropolitan area to expand onto those previously undeveloped lands. The years between 1850 and 1880 also saw the first major buildouts of Mexico's railroad networks. In 1872, a major line connected Mexico City and its core port city of Veracruz. And the longtime president Porfirio Diaz, who essentially served as a dictator during these times, made peace and invited foreign investors to Mexico. Many businesses and banks started to establish offices in the capital city. These trends together combined to kickstart Mexico City's urbanization and expansion. In 1859, Mexico City was home to about 210,000 people. By 1884, that would grow to 300,000. In the 1860s, we had the Second Mexican Empire, established by Mexican monarchists and the French. After yet another round of floods, the newly enthroned Emperor Maximilian initiated a search for a permanent drainage solution. This eventually led to the Great Drainage Canal, a massive canal and tunnel that would drain three of the lakes, keeping the waters from pouring into Lake Texcoco. The water would instead drain by gravity to the Texciquiac river, taking sewage with it. Work began in 1860, but progress was slow due to war and finances. After several stops and starts, the project was finally completed and inaugurated in 1900 by the aforementioned President, Porfiro Diaz. The final cost was about 16 million pesos. The canal got off to a rough start. A few months after its inauguration, it was inundated by a massive storm. However, over time, the canal and other drainage works steadily drained water out of the lake. Lake Texcoco Once spanned the entire basin at nearly 7,800 square kilometers. By modern days, the lake is now just about 16 square kilometers. And the city has spread out to take its place, building on top of the former lakebed soils of Lake Texcoco. Now we should discuss another one of Mexico City's water drinking water. The city had suffered freshwater scarcity since the days of the Aztecs. But it got more serious in the 1700s. As the basin's lakes either shrank or became polluted by livestock, the city struggled to bring in enough fresh water from its existing aqueducts. This only worsened after independence. With few available options at the ready, in 1847, the city government began drilling shallow wells to pump the groundwater below. Demand grew as the city's population did, and by 1886, there were over a thousand shallow wells in the basin. These early pumps pulled up the groundwater inside Lake Texcoco's legacy lake bed soils. This layer extends about 100 meters deep, and its soils are rich in clays. But removing the water surrounding those soil and clay grains will cause them to compress, in some cases up to 25 to 30% of their original volume. This causes the land to sink, a phenomena called land subsidence. Such subsidence is basically reversible. Reported signs of land subsidence date back to as early as 1900, when Roberto Gallo noticed that the entrance gates of the Grand Canal had popped out of their base. Because the base had sunk several centimeters on average, the subsidence was estimated to be about 5 centimeters per year. Realizing that the pumps cannot be the city's only water supply, the government built a modern water supply system in 1903 to transport water from the distant city of Xochimilco. But that was rated for just 350,000 inhabitants. Between 1910 and 1920, Mexico City's built up area expanded from 962 hectares to over 3,000. And by 1921, the city's population grew to 662,000 and then in 1930, over a million people. With those peripheral water sources exhausted, the city doubled down on groundwater. In the 1930s, pumping intensified with large electrically powered pumps and big pumping stations, bringing up more water than ever before. In the first few decades of the 1900s, the rural, focused agrarian policies of the Mexican Revolution restrained Mexico City's growth. Then, in the 1940s, new trends tipped the balance towards urbanization. The demands of World War II vastly accelerated Mexico's industrialization. A migrant worker program with the United States, the Bracero program, created a new class of migrant workers. That soon poured into Mexico City. And improving education and public health programs made the city a more attractive and safer place for people doing white collar work. If you wanted those jobs, then you had to go to Mexico City. But Mexico City's economic boom far outran the city's meager water infrastructure. As early as the 1920s, engineers had enough evidence to show that that the land was indeed sinking and that groundwater pumping was a major reason for it. Some downtown areas in the city basin experienced a peak subsidence rate of 46cm per year. Then the government passed restrictions on pumping in 1947. But the booming economy and a lack of easy answers meant those were thoroughly ignored. Capitalist and government interests also played a major role. The water pumps were originally made in the United States, but the Mexican government wanted to bring production to Mexico. Shutting off the pumps would have ended that industry. By 1970, the Mexico City metropolitan area counted over 9 million residents, many of whom lived in informal settlements surrounding the city. As construction could not keep up with growth, even intensive groundwater extraction could not satiate the city's thirst. In addition to the drilling, the city built water networks to bring in potable water from the nearby Kutzmala and Lerma river basins. It's a bit ironic, isn't it? For 300 years, the authorities spent huge resources to drain the basin's lakes. And they ended up succeeding so completely that that they then had to build infrastructure to bring water back into the city. The subsiding land affects people in the city unevenly. Neighborhoods built on softer ground experience broken pipes, cracked streets, and curving windows. Many of the worst affected neighborhoods tend to be less social, economically advanced, unable to keep up with the repairs. In response to these sinking soils, the city's engineers have adapted in interesting ways. Some lightweight houses might have shallow foundations that let them sink with the land. Tall buildings like the Latin American Tower have deep piles that go about 34 meters deep to hard rock or stable layers. This has kept what has long been Latin America's tallest building standing, despite land subsidence and several big earthquakes. Mexican engineers have innovated other interesting setups, including hybrid point penetration foundations that offer the flexibility to sink, but with extra support from the bottom layers. The threat to Mexico City's massive rapid transport system, the second largest in North America after New York City's, is also substantial. The metro traverses 140 miles of land, much of it sinking at different rates. Subsidence creates deep divots that can cause flooding or electrical shorts. It can create slope angles, which the system was originally not rated for leading to stalls. Engineers can add new material to adjust for the uneven elevation, but for how long this is uncertain. In 2021, a Metro overpass collapsed under the weight of a passing train, causing 26 fatalities and 50 plus injured, one of the worst such accidents in recent history. Was it caused by subsidence? The official report seems to point to construction deficiencies rather than the sinking land, but it's hard to imagine that the latter didn't play a role. But there has been a general lack of land subsidence information on that particular area. Land subsidence also negatively affects the water supply. A prominent example of this is the aforementioned grand drainage canal, which relied on gravity to drain water. But the land sank so much that it caused the drainage canal's flow to actually reverse, causing the city's sewage to come back up like vomit. Engineers had to build drainage outlets and an entirely new exit in 1960. The city is estimated to lose 40% of its water to leaks and breaks. The subsiding land runs the risk of exacerbating the water leaks as well as breaking sewage pipes, posing a dual threat to the already tenuous water supply. Such threats led to the city government to ban drilling in the city center in the 1970s, moving them to peripheral neighborhoods. The drills were also instructed to go deeper in an attempt to avoid compressing the shallow clay layers. Today, water comes from deep aquifer layers between 100 and 800 meters deep, which has somewhat slowed the rate of land subsidence. But some 58 to 70% of Mexico City's water supply still comes out of the ground, and the sheer amount of water extracted remains an issue, and the city continues to sink. Even now, it is hard to get a good estimate on the magnitude of the water pumping. Satellite readings from 2023 imply that about 1 to 13 cubic kilometers of water has been removed each year since 2014, or roughly about a billion gallons of water a day. Removing the dependency on groundwater will require extensive policy work to find new supply, find and fix leaks, and improve overall water conservation. There's talk about returning to the ways of Tenochtitlan, but I doubt that they apply for a city now 100 times larger. But even if Mexico City stops pumping water today, which is unlikely, land subsidence will continue on simply because the lakebed continues to compress. Some estimates find it will go on for as long as another 150 years and another 30 meters. So the sink is here to stay. Alright everyone, that's it for tonight. Thanks for watching. Subscribe to the channel. Sign up for the Patreon and I'll see you guys next time.