World overrun with undead jellyfish.
Well, in the jellyfish's defense, they aren't undead, just undying. Yeah these little buggers live forever.
I found the story here, but you don't have to read it. It's very self explanatory.
Due to something called transdifferation, the Turritopsi nutricula are capable of transforming their own cells back to immature polyp state. Yeah, they mutate themselves. UNDEAD MUTANT JELLYFISHES!!! They live out their lives over and over, going from young to old and back again. Because they reproduce but never die, their numbers are booming and they are now found throughout the world, instead of just in the Caribbean, where they originated. Yes, that's right. They are taking over the world, one squiggly mass at a time.
Luckily, no jellyfish is immune to toxic waste (at least not as of yet), so there is a we will avoid this specific apocalypse in favor of a flashier, if dirtier one.