Hydra are freshwater creatures capable of whole-body regeneration (AAAS). Those kept well seem to live indefinitely without showing any signs of aging (AAAS). Hydra are unique because their stem cells exist in a continuous state of renewal (UC Davis). “Heracles was lucky he wasn’t fighting something with the regenerative ability of the real Hydra, which can re-grow its entire body from a few hundred cells” (UC Davis; photos S Siebert).