PARIS: Four reactors at France’s Gravelines nuclear power plant were shut down late Sunday due to a swarm of jellyfish in the filter drums that pull in cooling water, operator EDF said on Monday.

The plant in northern France is one of the largest in the country and cooled from a canal connected to the North Sea. Its six units produce 900 megawatts of power each, or 5.4 gigawatts in total.

Reactors 2, 3, and 4 stopped automatically when the filter drums of the pumping stations became packed with with a “massive and unpredictable” swarm of jellyfish, and reactor 6 went offline shortly after, EDF said.

The entire nuclear plant has now 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.