What was Mexico City originally built on?

Question

What was Mexico City originally built on?

Answer 1

A lake

Answer 2

The city is built on top of a lake.  The Aztecs created man-made islands and had a great canal system.  The water areas were subsequently filled in over time and buildings were built.  The city is sinking little by little every year.  If you visit Xochimilco in the southern most part of the city, you can see how the city used to be like with the islands and canals.