Immortal Jellyfish

In R'lyeh lay sleeping. [Link]

The turritopsis nutricula species of jellyfish may, in fact, be the only immortal creature in the world. It is the only one we have discovered thus far.

This jellyfish achieves everlasting life through the process of transdifferentiation: a process by which one type of cell transforms into another type of cell. Usually, animals can only use transdifferentiation in order to regenerate organs or limbs (e.g. salamanders). Turritopsis nutricula, however, transdifferentiates throughout its entire life, enabling it to cycle from an immature polyp stage, to a mature adult, and then back to its polyp stage again.