Well, the city was built atop an aquifer or somesuch, and so it is sinking and become unstable. So the land will be unbuildable at some point.
Not exactly. Mexico City is built on the floor of a huge lake that was drained in the 16th century. It's subsiding, but it's bigger problem is that lakes tend to form in low spots, so the city is one big low spot. That can be a problem when it rains hard, for an extended period (the lake tries to come back).