One of the world’s largest nuclear reactors had to be shut down after a wave of jellyfish was huge enough to clog pipes that bring cool water to the plant’s turbines.

The operators of Oskarshamn nuclear plant in Sweden said they had to scramble reactor three after the animal clog. It was not the first time they had to shut down because of jellyfish. Reactor One was shut down in 2005 because of a jellyfish invasion.

Reactor three is the largest boiling-water reactor in the world. The three reactors at the Oskarshamn site are of the same technology as the Fujushima Daiichi plant that melted down in 2011 after the Japan tsunami.

Scientists say jellyfish causing shutdowns might be happening more often.

“It’s true that there seems to be more and more of these extreme cases of blooming jellyfish,” said Lene Moller, a researcher at the Swedish Institute for the Marine Environment, told the New York Post. “But it’s very difficult to say if there are more jellyfish, because there is no historical data.”