Last week, Sweden's Oskarshamn nuclear power plant, which supplies 10% of the country's energy, had to shut down one of its three reactors after a jellyfish invasion clogged the piping of its cooling system. The invader, a creature called a moon jellyfish, is 95% water and has no brain. Not what you might call menacing if you only had to deal with one or two.
En masse, jellyfish are a bigger problem. Coastal areas around the world have struggled with similar jellyfish blooms, as these population explosions are known.