A swarm of jellyfish that entered a cooling system paralyzed four reactors at the Gravelines nuclear power plant, French firm EDF announced Monday.

“These shutdowns are the result of the massive and unforeseeable presence of jellyfish in the filter drums of the pumping stations, located in the non-nuclear part of the installations,” an EDF spokesperson said in a statement to POLITICO.

The company is currently carrying out diagnostics and necessary interventions to restart the plant safely, the spokesperson added.

Three of the four reactors stopped working automatically late Sunday, with the fourth unit shutting down early Monday morning. The plant has six reactors in total, with each producing 5.4 gigawatts of power in total. The remaining two units are undergoing maintenance.

EDF — France’s main electricity generation and distribution company, via nuclear, hydropower, renewables and thermal power plants — said the incident does not pose a threat to the safety of the plant, its workers or the environment.

The plant is cooled off by water pumped from a canal connected to the North Sea. Beaches in the area have seen an increase in jellyfish in recent years due to rising sea temperatures, changes in salinity and overfishing, which reduces jellyfish predators.