Last week, Sweden’s Oskarshamn nuclear power plant, which supplies 10% of the country’s energy, had to shut down one of its three reactors after a jellyfish invasion clogged the piping of its cooling system. The invader, a creature called a moon jellyfish, is 95% water and has no brain. Not what you might call menacing if you only had to deal with one or two.

En masse, jellyfish are a bigger problem. “The [moon jellyfish swarm] phenomenon…occurs at regular intervals on Sweden’s three nuclear power plants,” says Torbjörn Larsson, a spokesperson for E.ON, which owns Oskarshamn. Larsson wouldn’t say how much revenue the shutdown cost his company, but noted that jellyfish also caused a shutdown in 2005.

Coastal areas around the world have struggled with similar jellyfish blooms, as these population explosions are known. These blooms are increasing in intensity, frequency, or duration, says Lucas Brotz, a jellyfish expert at the University of British Columbia.

Jellyfish are taking over the seas, and it might be too late to stop them