One of the facts about evolution that really blew my mind when I first learned about it was that whales had evolved from land mammals that had returned to the sea. Even though I had no formal training in biology beyond eighth grade, I was aware of course of the basic evolution story of how we all evolved from ocean organisms but the idea that at some point the process got reversed for some branches of the evolutionary tree and they become sea-dwelling creatures again just captured my imagination and I have posted about it before.
This animation shows how a reconstruction, based on the fossil record, of how it happened.



