A new study reveals the cause of Mexico City’s rapid sinking and sagging soil.

The ground in Mexico City is sinking at a rate of nearly 50 centimeters a year, and it won’t stop anytime soon, nor will it recover, according to Chaussard et al. in a new study.

Combining 115 years of ground-based and 24 years of space-based measurements, a team of U.S. and Mexican scientists has concluded that wide areas of land beneath the city are steadily compacting after water has long been removed from them. They predicted that the ground will continue to compact for about 150 years, adding up to 30 meters to what has already subsided several meters during the 20th century.

Unlike the subsidence seen in many other cities around the world, Mexico City’s subsidence does not seem to reflect the rate of local groundwater pumping, as one might expect. Instead, it reflects the constant compaction of the bottom of the ancient lake on which the city was built.

This lake was once Lake Texcoco, the site of the Aztec city of Tenochtitlan. As water was extracted, the underground water went deeper and deeper underground, and the lake bed, 100 meters thick and salty and rich in clay, remained dry and high. Since then, the very fine mineral grains have continually compacted, causing the ground to shrink and subside.

Such compaction is irreversible, the researchers report, and is responsible for the extensive cracks that damage buildings, historic monuments, sewers, gas and water lines in the city. The subsidence also opens up access to contaminated surface water, which could make access to clean water in the city, already difficult, even worse.

“Unless drastic measures are taken to manage water resources,” the researchers write, “things are headed for a double crisis-water and subsidence (subsidence).

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