When Darío Solano-Rojas moved from his hometown of Cuernavaca to Mexico City to study at the National Autonomous University of Mexico, the configuration of the metropolis troubled him. Not the grid itself, mind you, but the way the built environment seemed to tumble, like a surreal painting. “What surprised me was that everything was a bit twisted and tilted,” says Solano – Rojas. “At that time, I didn’t know what it was about. I just thought, ‘Oh, well, the city is so different from my hometown.’ ”
Different, it turned out, in a bad way. Resuming the study of geology at the university, Solano-Rojas met geophysicist Enrique Cabral-Cano, who was actually researching the surprising reason for this infrastructural chaos: the city was sinking – high time. It is the result of a geological phenomenon called subsidence, which usually occurs when too much water is extracted from the subsoil and the earth above begins to compact. According to new modeling from the two researchers and their colleagues, parts of the city are sinking up to 20 inches per year. Over the next century and a half, they calculate, areas could drop to 65 feet. The spots just outside Mexico City could sink 100 feet. This twist and tilt noticed by Solano-Rojas was just the start of a slow-motion crisis for 9.2 million people in the world’s fastest-sinking city.
The root of the problem is Mexico’s poor foundation. The Aztec people built their capital of Tenochtitlan on an island in Lake Texcoco, nestled in a basin surrounded by mountains. When the Spaniards arrived, destroyed Tenochtitlan and slaughtered its inhabitants, they began to drain the lake and build above it. Gradually, the metropolis that became modern Mexico City expanded, until the lake was no more.
And that triggered the physical changes that started the sinking of the city. When the lake sediments beneath Mexico City were still wet, the clay particles that made them up were arranged in a disorganized fashion. Consider launching plates in a sink, willy-nilly—Their random orientations allow a lot of liquid to flow between them. But remove the water – as the planners in Mexico City did when they first drained the lake, and as the city has since done by harnessing the soil as an aquifer – and these particles rearrange themselves to stack neatly, like plates in a cupboard. With less space between the particles, the sediment compacts. Or think of it like applying a clay mask. As the mask dries, you can feel it tighten against your skin. “It loses water and it loses volume,” says Solano – Rojas.
Officials in Mexico City actually recognized the sagging problem in the late 1800s, when they saw buildings sink and began to take action. This gave Solano – Rojas and Cabral-Cano valuable historical data, which they combined with satellite measurements taken over the past 25 years. By launching radar waves to the ground, these orbiters measure in detail – a resolution of 100 feet – how surface elevations have changed across the city.
Using this data, the researchers calculated that it will take another 150 years for Mexico City’s sediments to fully compact, although their new modeling shows that the subsidence rates will actually vary from block to block. (This is why Solano-Rojas noticed tilted architecture when he arrived.) The thicker the clay in a given area, the faster it sinks. Other areas, especially on the outskirts of town, may not sink much at all because they sit on rocks instead of sediment.
It sounds like a relief, but in fact exacerbates the situation because it creates a dangerous differential. If the whole city flowed evenly, that would be a problem, of course. But as some parts collapse considerably and others do not, the infrastructure that covers both areas sinks in some areas but remains at the same altitude in others. And it threatens to break roads, metro systems and sewage systems. “Subsistence in itself may not be a terrible problem,” says Cabral-Cano. “But it’s the difference in this speed of subsistence which really puts all the civil structures under different constraints.
It is not just Mexico City’s problem. Wherever humans extract too much water from aquifers, the land subsides in response. Jakarta, Indonesia is sinking up to ten inches one year, and the San Joaquin Valley in California has sunk 28 feet. “It goes back centuries. Human thought was that this [water] is an unlimited supply, ”says Manoochehr Shirzaei, a geophysicist at Arizona State University, who subsidence studies but was not involved in this new research. “Wherever you want you can punch a hole in the ground and vacuum it up.” Historically, groundwater pumping has solved immediate communities’ problems – keeping people and cultures alive – but created much longer-term disaster. A study done earlier this year found that by 2040, 1.6 billion people could be affected by sagging.