Just 5 millimetres wide, the tiny Turritopsis dohrnii has discovered how to cheat death. More commonly known as the immortal jellyfish, it has been silently invading oceans all over the world with its ever-increasing population—due to the fact it can age backwards. The jellyfish’s reproduction cycle involves the meeting of free-floating sperm and eggs, which then settle on a hard surface and form a blob-like polyp, which slowly matures. Most mature jellyfish species die soon after reproducing, but the Turritopsis is able to transform from back into a polyp and restart life anew, inverting their ‘umbrella’ and absorbing their tentacles. This can only be done in an emergency such as starvation, physical damage, or temperature or salinity change, but the cycle can be repeated indefinitely, rendering the Turritopsis immortal. Remarkably, their cells are completely transformed in the process. Biologist Stefano Piraino thinks that they’re able to “switch off some genes and switch on [others], reactivating genetic programs that were used in earlier stages of the life cycle.” However, researchers have dismissed ideas that the species could hold the key to anti-aging drugs—and maybe that’s for the best. If the Turritopsis can spread this rapidly through the world’s oceans, then I don’t think immortality would very healthy for humans.