A box jellyfish's sting can stop a human heart within minutes

Chironex fleckeri's venom is potent enough to cause fatal cardiovascular collapse within two to five minutes of a sting, among the fastest-acting venoms of any animal. Its tentacles, up to three metres long, are lined with millions of microscopic harpoon-like stingers that fire into skin on contact.

— James Tibballs, Australian venomous jellyfish, envenomation syndromes, toxins and therapy — Toxicon, 2006