From the beaches of the Mediterranean to New York's Long Island, swimmers are encountering huge swarms of jellyfish in the water and on the shore.
There are always jellyfish in oceans in August, but this year in some places they arrived early. Scientists say there are more than ever before, and they are appearing in places where jellyfish have rarely been seen.
Competitors in the recent New York City triathlon swam through jellyfish in the Hudson river, and in the surf off Long Beach, thousands of swimmers have already been stung this summer.
Marine biologists say the population explosion is not just a nuisance, but sign of the declining health of our seas.
"Jellyfish are really just a symptom of oceans in crisis," says Margo Stiles, marine scientist.
Experts blame the worldwide invasion on several factors, including global warming, the overfishing of predators like tuna, and pollution - primarily from the runoff from fertilisers to keep lawns green.
It turns out that jellyfish thrive in coastal waters where oxygen levels have been depleted by that run-off.
During a recent expedition to a lagoon on the coast of Spain, scientists from the group Oceana were surprised to find a massive concentration of jellyfish.
"There were more than 20 million jellyfish in that lagoon right in front of the vacation homes," says Ms Stiles, "which is good for learning more about them, but not so good if you own one of those homes."
Marine biologists say the scourge - which in some places is overwhelming commercial fish stocks - will disappear in the fall as the water gets colder, only to return next summer... possibly in bigger numbers.
CBS