A nuclear plant in northern France was temporarily shut down on Monday after a swarm of jellyfish clogged pumps used to cool the reactors, energy group EDF said.

One of the largest French nuclear power plants was temporarily shut down after a swarm of jellyfish entered the water intake systems used to cool the coastal reactors, on Monday its operator said.

A massive jellyfish invasion clogged the cooling intake systems at France’s Gravelines Nuclear Power Station, forcing several reactors offline in a rare, non-nuclear shutdown.