Australia's biggest attraction is the Great Barrier Reef, one of the wonders of the natural world. The reef covers about 300,000 square kilometres and is home to many amazing animals and plants. Although you're allowed to swim and dive on the Barrier Reef, you have to be careful as some of its residents are not very friendly! Stonefish The stonefish looks like a stone, just like the reef that it lives on. It also likes to sit on sandy It has spines, though, the seabed. that it uses to defend itself. They are very sharp and contain a nasty venom which can be very painful and sometimes lethal. To avoid the stonefish's nasty sting, you mustn't touch the reef while swimming and you should make sure you always wear shoes or sandals when walking in shallow, sandy water Irukandji jellyfish These tiny jellyfish are the size of your thumbnail and impossible to see in the water. Don't be tricked, though, by their size! They are one of the most poisonous animals on the planet and a sting from one of them causes terrible symptoms that unless treated properly might kill you.