Reports coming out of Orange County, in Southern California, suggest that a rare giant jellyfish, only recently discovered, is invading beaches and stinging swimmers. On July 4th, swimmers in Laguna Beach, a beach town famous as a setting for great television, came ashore with hefty stings on their bodies and dark, odd membranes sticking to them.

So what is this weird plague?

The black jellyfish (Chrysaora achlyos, also called the black sea nettle) is huge, with a bell (that’s the dome-shaped part of the body) that can reach three feet across. But it’s the rest of the body that’s so scarily big: its arms, described by the Monterey Aquarium as “lacy and pinkish” can reach 20 feet long, and its tentacles can by nearly 8 feet long. It was only officially named and described in 1997, though it can be seen in photographs as far back as 1926. (It’s clear in photographs, since it’s the only dark-colored jellyfish in that part of the Pacific Ocean.)

This jellyfish invades the coast periodically, dependent, we think, on ocean temperature (its invasions coincide with El Niño and La Niña). Human influence on the oceans may also be a factor; increased levels of organic matter, like fertilizer from farms, may attract or feed zooplankton, which the jellies follow and eat. We have no idea what it does or where it lives when it’s not washing up on the shores of wealthy Southern California beach towns, but it comes in large numbers every decade or so. It was first noticed in 1989, then in 1999, and now this year.

It eats zooplankton and sometimes other jellyfish, we think; we know hardly anything about this creature. We do know that its sting is painful but not debilitating or deadly to humans, the pain only lasting about 40 minutes and having no known lasting effects. That’s likely due to its diet; jellies that feed on larger or more complex animals like fish tend to have stronger stings. The stings of some jellyfish, like a few species of box jellyfish, can be fatal to humans. Not the black sea nettle, thankfully.