The demand for water in this metropolis of over twenty million people is enormous. Although much of the city's water supply is piped in from beyond the valley, the aquifer beneath the old lake bottom continues to be exploited. As a result, much of the city is literally sinking as the spongy soil subsides. In the city's historic center, one can see many buildings which are tilting at crazy angles or which have sunk below street level.
The summer rains should replenish the aquifer, but, because the valley has been paved over with construction, the rainwater has nowhere to go. The drainage system is inadequate, and the many of the city's streets become canals. I remember one evening when I went with Alejandro from my apartment to his family home. It had been raining heavily, and we did not realize until it was too late that the highway to get to his house was under water. By the time we found a way to get to his home it was the wee hours of the morning.
Below is a video that a friend of Alejandro's sent him a couple weeks ago. It shows flooding on the east side of the city. Just last night there were heavy rains that flooded many streets again. Alejandro called while driving home from work, and described the policemen standing mid-calf in water trying to direct traffic.
As is so often the case, human efforts to modify nature have a tendency to backfire.