One type of jellyfish can kill a fully grown human.
The box jellyfish is thought to be the most venomous marine animal in the world. It has a cube-shaped body and is typically found in waters around Australia and the Indo-Pacific. Its tentacles are covered in poison-filled darts. Only a few types of box jellyfish have venom that's lethal for humans, but a person who gets stung by one of these jellies can go into cardiac arrest or die within minutes.