What is Mexico City built on top of?

A drained lake. The Aztec capital Tenochtitlan was built on an island in Lake Texcoco, reached by causeways, with floating gardens and canals. The Spanish took the city in 1521, built their own on the ruins, and then spent four centuries draining the lake because a European-style city on a lake floods.

The consequence is that Mexico City sits on soft compressible clay, and it is sinking - in places by close to half a metre a year, and by more than ten metres cumulatively over the last century. Buildings tilt and separate visibly; the Metropolitan Cathedral has sunk unevenly enough to need internal engineering to keep it standing, and pavements ripple.

The reason is groundwater. The city pumps its water from the aquifer beneath it, the clay compacts as the water leaves, and the ground goes down - which then breaks the water and sewer pipes, which leak, which wastes water, which requires more pumping. The soft ground also amplifies earthquakes badly: the 1985 quake had its epicentre 350 kilometres away and destroyed the centre of the city, because the old lakebed shook like jelly at exactly the frequency of mid-rise buildings.