Why is Mexico City sinking?
Mexico City is drinking itself into the ground
Mexico City is sinking. Many of Mexico City’s buildings are seriously leaning because of land subsidence. Mexico City’s water balance has an aquifer deficit, which has caused the drying up of the heavily saturated clay of the former Lake Texcoco (on which the city rests on) and has led to land subsidence.
The city has sunk more than 10 meters in the last 60 years because 70% of the water people rely on is extracted from the aquifer below the city. Currently, the water table is sinking at a rate of 40 cm per year. As the city population grows and water demand increases, the problem only gets worse and Mexico City keeps sinking.
If you pump water out faster than the rainwater can trickle back in, the aquifer runs out. Mexico City already pumps out water over twice as fast as it can be replenished, and the population of the city continues to grow.
All that pumping is literally sinking the city. As the city drains water from the aquifers, empty space is left in its wake. The ground, now without structural integrity, sags into that void. In some places, Mexico City is subsiding as much as 38 cm per year. For comparison, the famously sinking Italian city of Venice is sinking at a rate of less than half an inch per year. Over the last century, experts estimate, Mexico City has sunk around 10 meters.
Soft lake sediment underneath the city has made it vulnerable to soil liquefaction during earthquakes. Because the sinking is not even across the city, buildings tilt to the side, and pipes break: as a result, repairs and maintenance are expensive.
The sinking of Mexico City has come to a point where the Zócalo, the main square in the historic city center, is at a lower elevation than Lake Texcoco. All over the city (most iconically in the historic center) buildings and churches lean like drunken men, the land having made an uneven descent into the earth beneath their foundations.
The city’s cathedral, which took more than 200 years to build, has a leaning chapel and bell tower, propped up by stone wedges to stop the whole thing crumbling down.
The Gilded Angel of Independence – a local tourist hotspot and national landmark – was built with nine shallow steps leading up from the street below. As the surrounding area has sunk, an extra 14 large steps have been added as the angel is increasingly left marooned above a vanishing city.
Slanted buildings leer menacingly over pavements, their doors and windows no longer in alignment with their friends as if crudely displaced from a grotesque theme park funhouse.
Terraced streets built on level ground now undulate, with wavy gables crowding up against each other in parts, and pulling away in others, while city-dwellers struggle up hilly pavements where once the path was flat.
Giant sinkholes open up without warning, swallowing parts of roads. Cracks open in the street, and buildings collapse or become too unsafe to inhabit. It’s a chronic threat in Iztapalapa, a poor neighborhood in the city’s southeast, home to some 2 million people.
But what happened? Why is Mexico City sinking?
The Aztecs founded the city of Tenochtitlán in the middle of a lake in 1325. Then a small island on the western side of Lake Texcoco seemed like a pretty good place to be. The Aztec Empire became the most formidable force on the continent. The island city Tenochtitlán was smattered with canals, causeways, and dikes. The channels Aztecs built provided natural irrigation and water management while floating gardens called chinampas kept the city fed as it grew to an astonishing size. By the time the Spaniards arrived there, the city had approximately 300,000 residents.
After the Spanish seized Tenochtitlan, they drained the lake, destroyed the Aztec city, and built Mexico City. Where the Aztecs had constructed dikes and channels to live in harmony with the lake, the Spaniards covered them over to build roads and increase the city’s size. They started draining the lake and cutting down the forests on its shores, making the city more susceptible to intense flooding.
Groundwater overdraft is depleting the aquifers beneath the city, causing it to sink. This has been ongoing since the mid-19th century. Although the aquifers are replenished, the rate of recharge is only about 50% the rate of groundwater extraction.
Mexico City is built on the basin of ancient Lake Texcoco, the largest of several lakes of which only remnants remain, most of them drained to control flooding. Much of the ancient endorheic lake basin was fed from groundwater aquifers.
The consequences of the draining, completed in the 20th century, were enormous: the area turned semi-arid and currently the city experiences water scarcity. In addition, soft lake sediment underneath the city has made it vulnerable to soil liquefaction during earthquakes.
Because the sinking is not even across the city, buildings tilt to the side, and pipes break: as a result, repairs and maintenance are expensive.
Mexico City filled up like a cup in the rainy season. But the Spanish continued to drain the lake system, and the city took deeper root atop silty-clay lakebed. Texcoco and all the other lakes are gone, with the exception of a few marshlands and a region of canals in the south of the city. The bowl-like depressions where the lakes once stood had no natural exit for water, and the denuded forests, whose soils once acted as sponges for flood water, no longer served as a buffer between water and people.
Mexico City was built on bad choices, given it sits on an unstable crust of clay and a swath of lava rock. Clay and lava, nearly all paved over—it’s the worst possible combination.
Mexico City suffered major floods in 1555, 1580, 1604, and 1607 before was proposed moving the capital to dry land in 1630. But after deliberating on this for a while, the authorities decided that the answer was no, and the flooding continued, with more serious deluges in 1645, 1674, 1691, 1707, 1714, 1724, 1747, and 1763.
One flood was so severe that the entire city was submerged for five whole years from 1629 – yet the city lived on, gasping for air and expanding further across the lakebed between downpours.
By the 20th century, most of the lake had been drained, and flooding became the least of the city’s worries. As it grew and grew, and poorer migrants from the surrounding country arrived in search of economic opportunity, Mexico City grew thirsty.
As more and more water was withdrawn, subsidence began. Subsidence was first noticed in 1891 in the old part of the city but not studied until 1925. In 1948 it was proved that groundwater withdrawal was causing subsidence.
In 1954, the overexploitation was sufficiently serious that pumping was banned in the city center and wells were moved to the north and south of the basin. Subsidence is now stabilized in the city center but is still a major problem in most parts of the metropolitan area.
The city is built on two different geological foundations. Some of the ground underneath Mexico City is volcanic soil, which was fertile and used by the Aztecs for growing crops. It was also handily water-absorbent: moisture would soak in and flow to underground aquifers easily, without damaging the structure of the soil.
But when developers built on the volcanic soil and covered it in concrete and asphalt, water could no longer get through to the soil and filter through to the aquifers on which the city relies.
And other parts of the city sit on clay. This, unlike the volcanic soil, can’t absorb the water, merely sandwiching it between layers of clay-like cream between layers of pastry. When the cream is sucked out, the layers of pastry crack and collapse, falling on top of one another.
Now, without the lakes, Mexico City turned to the groundwater for drinking water. The groundwater was and still is, stored in the relatively shallow aquifers that lie beneath the lake beds.
Land subsidence has been caused by groundwater overexploitation during the last hundred years and has been up to 9 meters, resulting in damages to buildings, streets, sidewalks, sewers, stormwater drains and other infrastructure.
Desperate for water in a lake basin devoid of a lake, the city has tapped into the clay soil while covering over the useful volcanic soil. And as the city is built on a mixture of both geologies, it has sunk in an uneven, mismatched way, causing dangerous fissures, cracks, and the bizarre phenomenon of wavy, undulating streets.
The collapse in the central region of the city reached 10 meters at the end of the 20th century, while in the sub-basin Chalco-Xochimilco, it reached 7 meters.
But not all of Mexico City was built on the lakebed. Southwest of the city center is a region now known at the Pedregal, which rests on the hardened lava flow.
The population of Mexico City tripled between 1950 and 1975. Pedregal land, which up to this point was mostly regarded as an uninhabitable rock heap, was cheap. Developers saw a gold mine and subdivided large lots into high-end residential communities. Hundreds of families rushed the Pedregal over a short period, built their houses, and started neighborhoods.
The rapid growth of the southwestern part of Mexico City would have unforeseen consequences for the entire metropolitan area. Between the mid-1950s and mid-1980s, nearly the entire dark swath of volcanic rock that once covered the Pedregal region—comprising roughly 8,000 hectares, was swallowed up by streets and buildings. The unique ecosystem was almost entirely paved over.
In fact, while Mexico City residents endure months of regular flooding during the rainy season in some parts of the city, virtually none of that water makes it underground. That’s because rapid urbanization has sealed up any permeable surfaces in the city with the pavement. In short, the city’s pores are clogged.