Four reactors at France’s Gravelines nuclear power plant, located along the English Channel, were temporarily shut down over the weekend after a swarm of jellyfish threatened to clog critical water intake systems.

One of the largest nuclear power plants in Europe was forced to shut down after some unusual visitors. Gravelines nuclear power plant in northern France was fully shut down on Monday after a ‘massive and unpredictable’ swarm of jellyfish entered the plant’s pumping stations.

Located on the Channel coast near Dunkirk, Gravelines is the largest nuclear power plant in western Europe, with six 900 megawatt reactors.