90 people at a single beach in Germany were treated for jellyfish stings in just three days, amid a surge of the creatures in European waters.
Regional newspaper Ostsee Zeitung said "tens of thousands" of lion's mane jellyfish washed ashore at the weekend.
The lion's mane is the largest jellyfish species, and while its sting is rarely fatal it often requires medical attention.
One of the 90 people affected had a severe allergic reaction.
Germany's lifeguard association said that on Tuesday only a few specimens had been counted.