The Gravelines nuclear power plant in northern France has been shut down after a swarm of jellyfish entered the filter drums that pull in cooling water, according to its state-owned operator, EDF.

The plant in northern France is one of the largest in the country and cooled from a canal connected to the North Sea.

Four reactors were affected by the incident, which occurred late on Sunday and led to the entire plant being shut down. Reactors 2, 3 and 4 stopped automatically when the filter drums of the pumping stations became packed with a “massive and unpredictable” swarm of jellyfish, and reactor 6 went offline shortly after, EDF said.

The entire nuclear plant, located on the French coast between Calais and Dunkirk, has temporarily halted production as the other two units are offline for planned maintenance, EDF data showed.

Several species of jellyfish are native to the North Sea and are often seen around the shoreline in the summer when the waters are warm.

The event did not affect the safety of the facilities, staff or the environment, EDF said.

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The plant’s six units produce 900 megawatts of power each, or 5.4 gigawatts in total.