An Even Descent

Sony Salzman
BU News Service

Deep underground, below the bustling streets of Mexico City, engineers navigate a complex system of struts and support pylons in an attempt to stop the oldest cathedral in the New World from sinking into the earth.

Their attempts are in vain. All of Mexico City is sinking at an astonishingly fast rate. Venice, Italy – arguably the world’s most famous sinking city – is loosing less than an inch per year. In the same time frame, Mexico City descends more than three feet.

The city rests on top of a giant underground lake. As 20 million thirsty citizens pump water up from the underground aquifer, the city sinks a little lower into the ancient lake bed.

This creates an uneven cityscape and a perilous water situation, but residents are particularly concerned with the preservation of the Metropolitan Cathedral, a splendid 500 year old church that can be compared to Notre Dam. When Hernán Cortés arrived in Mexico in 1519, he came upon a magnificent Aztec city constructed smack in the middle of a lake. He conquered the city, filled in the lake with dirt, and broke ground on the first version of the cathedral with stones from the Aztec temple.

Mexico City ©Francisco Diez

Now, Metropolitan Cathedral rests in the heart of Mexico City. In the 1990s, the building had tilted so much that it cracked down the middle. The problem is that the lake bed beneath falls away unevenly, so that at one point one side of the building’s foundation sat more than 5 feet lower than the other size.

So far, the best solution engineers have come up with is to build support structures underground that level the building … so that the entire building sinks a the same rate. Clearly, it would be ideal to stop the sinking all together, but Mexico City inhabitants have a big problem on their hands that won’t go away with a quick engineering fix.

And the crisis is quickly getting worse. In fact, the sewage draining system that pushes waste away from the city center has reversed its course. Now, in times of heavy flood, sewage actually heads back to downhill towards Mexico City.

There have been many proposed solutions. One idea is filter the dirty water and re-inject it into the ground. This would keep the water table underneath the city relatively constant and prevent further sinking. Another solution is to import water from other regions. Both of these options are extremely expensive, and unlikely to take root any time soon. Mexico City’s sinking problem requires the attention of the world’s top engineers and a huge upfront investment. But in the meantime, the city will continue to sink … hopefully at an even keel.

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