These jellyfish have a comparatively harmless sting, but they have shut down a nuclear power plant in Sweden.

The power plant, located on the Baltic coast, had to be shut down to prevent overheating after an onslaught of jellyfish. The jellyfish clogged up the intake pipes for the cooling system. The pipes collect water from the Baltic Sea to cool the reactor.

This is not the first invasion of jellyfish to shut down a reactor at this power plant. In an onslaught of jellyfish in 2005, another reactor had to be shut down.

These moon jellyfish, as they are known, are currently experiencing a bloom cycle—a surge in population. In large numbers they can cause a lot of damage. In this case, they have knocked out five percent of Sweden's power.