It’s one of the world’s largest nuclear reactors but small, brainless, water based jellyfish shut it down.
Workers at the Oskarshamn power plant in Sweden say millions of small moon jellies combined to clog the reactor's intake pipes.
Popular Science reports the shutdown didn’t last long and were caused by what’s termed as a "bloom" cycle.
The bloom causes massive amounts of the animals to be born at once and the one marine biologist said “more and more of these extreme cases of blooming jellyfish" are happening.
Moon jellies love areas that have been depleted by overfishing and prone to large algae blooms.
The biologist told Phys.org "they don't care if the oxygen concentration is low," instead thriving in the conditions.
A similar situation happened last year at the Diablo Canyon plant in California but that issue came from long stringy looking creatures called sea salp.