There is an excellent clue in http://www.snpp.com/episodes/2F06.html:
Homer: Marge? Kids? Everything's going to be just fine.
No go upstairs, and pack your bags...we're going to start a new
life...under the sea.snip
Homer: Under the sea, under the sea,
There'll be no accusations, just friendly crustaceans
Under the sea!
snipMarge: Homer, that's your solution to everything: to move under the sea. It's not going to happen!
Homer: Not with _that_ attitude!
So, the Simpsons live under the sea!
The best part of that episode is the badly edited interview of Homer that appears on "Rock Bottom". Due to the editing, the hands on the clock in the background jump around like crazy!
Aaron Hirshberg