Before the Spanish colonization in 1525, there was a huge lake that surrounded Mexico City's downtown. The Spanish empire drained the surrounding lake over centuries and expanded Mexico City onto the new land.
Lake Xochimilco is the only lake that survived but it is disappearing due to urban sprawl and no government action.
Currently, there are a few channel crossings that the local 'Xochimilcas' use as another means of transportation to avoid traffic jams, the boats they use are called 'trajineras'.