Why do dead whales explode?

Cows will do this too (Warning, gross picture):

http://pics.ww.com/v/jacques/trips/us2/dscf0622.jpg.html

Basically any animal that's got a pretty good seal going and that is in a prolonged state of decomposition will eventually puncture like a balloon at its weakest point. The reason is simply the gases created by the bacteria that decompose the body have to go somewhere, the better the seal the bigger the eventual bang.