[m0llusk]

It's a modern problem. Below the current city are the remains of several other cities that previously sank into the muck. The problem only became difficult to manage when we stopped rebuilding over sunken structures and started continually investing in extremely complex and heavy structures that we are hesitant to write off.

[shjake]

Problem is that without those old buildings there are no real practical reasons to live in Venice or build anything on top.