A relative of jellyfish, the rice-sized freshwater creatures known as hydra are, at first glance, rather basic – all tentacles and mouth, with lives dedicated to nabbing passing prey. But, as scientists have gradually been discovering over centuries, these simple organisms have such a unique capacity for regeneration that they’re considered biologically immortal. As this video from the science documentary series Deep Look explores, the hydra’s ability to rebuild itself is so powerful that the animal can even reform after being essentially blended at a cellular level. Providing astonishing high-definition glimpses of its microscopic world, this short details why the secret to the hydra’s invincibility is in its stem cells, and how scientists hope to harness its qualities to benefit humans.