In warm coastal waters around the world, swimmers can often spot large groups of jellyfish pulsing on the seafloor. It is best to avoid areas that upside-down jellyfish inhabit: getting close can lead to irritating stings, even without contact. Researchers have taken a close look at the cause of this mysterious ‘stinging water.’ Now, a team of scientists reports on the culprit — a toxin-filled mucus the jellyfish release into the water.