Everyone loves to fantasize about the idea of immortality. For this jellyfish, there's no fantasizing, it's a reality. Turritopsis nutricula is a species of jellyfish found to possess the amazing ability of transdifferentiation, they can revert back to the polyp stage and begin their life cycle again.
Other animals use transdifferentiation for equally amazing things like regrowing limbs and organs, but this is the only creature that has been found to have this amazing ability to get a fresh start on life.
While cool, there's a drawback for everything else. These jellyfish are native to Caribbean waters, but are now found to be spreading much further out than their native range. I say that a good old stake to the heart stops most immortals from getting out of hand. What's that? Jellyfish have no hearts? We are truly doomed.