Box jellies (not jellyfish, mind you), are perhaps the sneakiest of all creatures that can kill you.
Their bodies are almost perfectly transparent, and their tentacles are 16 feet (5 meters, as Aussies would say) long and all but invisible. This stealthy floating jelly monster is so loaded with stingers, that contact with one causes immediate death--one's heart simply stops beating from the shock. And bonus, they only float near the shore in the middle of summer... when everyone wants to swim. Yeah, it's not a great bonus.
So to recap: box jellies are invisible, huge, deadly, and available during the summer. Great...
It was my friend Joeline that first told me about them when I visited her in Melbourne.
At the time, I found it odd that anyone would advocate swimming when it could possibly end in death, but when you live somewhere like Australia, with lots of natural hazards, you have to accept that anything fun comes with a few risks. If you lived your life trying to avoid all danger, you'd be wearing a hazmat suit every day and hiding under your bed.
I like to think that Australians are as outgoing and fun as they are because they fully comprehend that any day could be your last, so you best enjoy the time you have. I mean, sure you could be stung by a box jelly and go immediately into cardiac arrest, but that's no excuse for never enjoying a swim.
Check out this great National Geographic article about box jellies: http://animals.nationalgeographic.com/animals/invertebrates/box-jellyfish/