Oh that's easy - the Zanclean flood.

What is now the Mediterranean sea used to be a land mass full of grass fields. A few million years ago, the Strait of Gibraltar gave way and hundreds of millions of cubic feet of water poured in. It's debated how long it took to fill up the Mediterranean. Could've been weeks, could've been decades. But for whatever period it was, it'd be nuts to sit there and watch it all unfold.