Jellyfish are free-swimming non-vertebrates found in oceans all across the world.
Consisting of a gelatinous bell with trailing tentacles capable of stinging prey, they can pose a threat to careless divers. While jellyfish stings are in most cases relatively harmless—although painful to Humans—some can be extremely serious, to the point of being lethal.
In ancient Greece, misthios Kassandra often encountered jellyfish in the Mediterranean Sea whilst diving to underwater locations. They proved harmless as the jellyfish didn't use their defense mechanisms.