Mexico City’s Metro System Is Sinking Fast. Yours Could Be Next

With its expanse of buildings and concrete, Mexico City may not look squishy—but it is. Ever since the Spanish conquistadors drained Lake Texcoco to make way for more urbanization, the land has been gradually compacting under the weight. It’s a phenomenon known as subsidence, and the result is grim: Mexico City is sinking up to 20 inches a year, unleashing havoc on its infrastructure.

That includes the city’s Metro system, the second-largest in North America after New York City’s. Now, satellites have allowed scientists to meticulously measure the rate of sinking across Mexico City, mapping where subsidence has the potential to damage railways. “When you’re here in the city, you get used to buildings being tilted a little,” says Dari?o Solano?Rojas, a remote-sensing scientist at the National Autonomous University of Mexico. “You can feel how the rails are wobbly. Riding the Metro in Mexico City feels weird. You don’t know if it’s dangerous or not—you feel like it’s dangerous, but you don’t have that certainty.”

In a recent study in the journal Scientific Reports, Solano?Rojas went in search of certainty. Using radar satellite data, he and his team measured how the elevation changed across the city between 2011 and 2020. Subsidence isn’t uniform; the rate depends on several factors. The most dramatic instances globally are due to the overextraction of groundwater: Pump enough liquid out and the ground collapses like an empty water bottle. That’s why Jakarta, Indonesia, is sinking up to 10 inches a year. Over in California’s San Joaquin Valley, the land has sunk as much as 28 feet in the past century, due to farmers pumping out too much groundwater.

A similar draining of aquifers is happening in Mexico City, which is gripped by a worsening water crisis. “The subsurface is like a sponge: We get the water out, and then it deforms, because it’s losing volume,” says Solano?Rojas. How much volume depends on the underlying sediment in a given part of the city—the ancient lake didn’t neatly layer equal proportions of clay and sand in every area. “That produces a lot of different behaviors on the surface,” Solano?Rojas adds.

Subsidence rates across Mexico City vary substantially, from 20 inches annually to not at all, where the city is built atop solid volcanic rock. This creates “differential subsidence,” where the land sinks differently not just square mile to square mile, or block to block, but square foot to square foot. If a road, railway, or building is sinking differently at one end than the other, it’ll destabilize.