Mexico City is a huge city, the second biggest in the world in fact, and it is a city that has been built over the Aztec capital of Tenochtitlan. The problem is that it is actually built over a drained lake. Because relatively recently there was a lake present in that area, the soil is very soft and labile, and there's lots of groundwater reserves bellow it. By taking out the water of the groundwater reserves, big spaces are opening up inside the ground, and the soft sediments from above can not sustain the weight on top of them, so the land starts to sink.
