Chironex fleckeri

The Australian box jellyfish is the most feared animal on this list, and the reason northern Australian beaches post stinger warnings. Its pale, boxy bell reaches about 30 cm and trails up to 60 tentacles in four clumps, each up to 3 m long, and it is found most often in summer in inshore waters. It lives along tropical coasts from Western Australia through the Northern Territory to Queensland; in one Northern Territory study, 92% of stings came during the stinger season of October 1 to June 1. Most stings are not life-threatening, but people who die usually go into cardiac arrest within minutes. Flood the sting with vinegar for at least 30 seconds and get medical help at once.