Thousands of aggressive flying squid with razor-sharp beaks have invaded the shallow waters off San Diego, California.
The squid, which can weigh up to 100lbs, came up from the depths last week and swarms of them attacked unsuspecting divers. Some divers reported tentacles enveloping their masks and yanking at their cameras and gear.
Stories of too-close encounters with the creatures have chased veteran divers out of the water and created a whirlwind of excitement among the rest, who are torn between their personal safety and the once-in-a-lifetime chance to swim with the deep-sea giants.
The Humboldt squid are native to the deep waters off Mexico, where they have been known to attack humans and are nicknamed "red devils" for their rust-red colouring and aggression. Those who dive with them there put bait in the water and sometimes get in a metal cage or wear chain mail to avoid being lashed by tentacles.
"I wouldn't go into the water with them for the same reason I wouldn't walk into a pride of lions on the Serengeti," said Mike Bear, a local diver. "For all I know, I'm missing the experience of a lifetime."
The squid are too deep to bother swimmers and surfers, but many divers say they are staying out of the surf until the creatures clear out.
Scientists are not sure why the squid, which generally live in deep, tropical waters off Mexico and Central America, are swarming off the Southern California coast - but they are concerned.
In recent years, small numbers have been spotted from California to Alaska - an alarming trend that scientists believe could be caused by anything from global warming to a shortage of food or a decline in the squid's natural predators.