A nuclear power plant in northern France hit by a jellyfish invasion was getting back online on Wednesday, operator EDF said, with service restored at the first of four closed reactors.

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.

Last weekend, a surprising and uncommon issue arose at one of the biggest nuclear power facilities in Europe. It was not a storm, a technical breakdown, or a security threat. Instead, it was an invasion of jellyfish that forced part of the facility to stop operations.