Instructables.com has details on how to make your very own erupting volcano cake. Clearly, this is a very good idea and nothing could possibly go wrong.

/Variations
Jurassic Boiling Tar Pit Cake: I bet I could use the volcano cake as a template to make a boiling tar pit of icing with dinosaur-cakes helplessly drowning in it. The biggest challenging could be finding the black food-dye for the tar.

Or maybe an Evolution Cake with edible fish-with-feet emerging from a bubbling primordial ooze-icing as the first land creatures.

Other ideas?