People have been banned from swimming on beaches in Europe after hundreds of people got stung by jellyfish.
Swarms of the creatures have invaded the coast off Spain and Italy, forcing loads of holidaymakers to steer clear of the water.
Reports say at least 30,000 people have been stung since summer began, and 19,000 of those attacked were in Spain.
Experts say the hot, dry weather is encouraging the creatures to stray from North Africa where they usually live.
They have been on Earth for 650 million years
They have no brain, heart or bones
Sensors tell them which direction they're going in
Their bodies are 95% water
The Box jellyfish is very dangerous and can kill you
That's because the water at the moment is warmer and saltier than usual because there is less water flowing into the sea from rivers. So jellyfish are being washed closer to the coast than usual.
Marine biologist Francesc Peters said Mediterranean jellyfish do not have very powerful stings, though it is possible to suffer an allergic reaction from them.
People who have been stung are advised to wash the sting in salty water and cool it with ice inside a plastic bag.