Box jellyfish.
You don't see it. You don't feel it at first. But the moment those tentacles touch your skin, a clock starts. And you have about four minutes.
That's not a dramatic estimate. That's the medical window before cardiac arrest.
The box jellyfish isn't the largest animal in Australia. It isn't the loudest. It doesn't have teeth or venom sacs you can see. It has 60 tentacles, each up to 3 metres long, lined with 5,000 stinging cells that fire the instant they detect pressure.
Contact is