Swarms of giant lion’s mane jellyfish whose tentacles can stretch 120 feet and unleash painful stings even 25 days after they’re dead are invading New England’s quaint beaches in a scene straight out of a horror flick.

Cities and agencies in Massachusetts have issued high alerts over the large number of terrifying giant 6-food-wide jellies infiltrating the area.

“Keep children and pets away from stranded jellyfish. … Do not touch jellyfish or detached tentacles, even if they appear dead,” warned the beach town of Beverly in a public-safety alert last week.

The jellyfish swarm is the biggest of its kind since 2020, experts told the Boston Globe.

Lion’s mane are some of the biggest jellies in the world, with the largest ever recorded found off Massachusetts with tentacles stretching 120 feet, according to Smithsonian Ocean.

The species also is infamous for how long it can remain dangerous: Its tentacles have been known to still deliver painful stings up to 25 days after the creatures die.

People on social media are reporting the jelly swarms from the Nahant peninsula to Salem, Mass.

“Right now, Cape Cod is inundated with Lions Mane Jellyfish! Multiple thousands of them have gotten swept into the marsh with the tides,” lamented Great Marsh Kayak Tours on Facebook.

The Boston Center for Youth and Families warned in a post, “Jellyfish are visiting the beach at BCYF Curley Community Center in #SouthBoston and have stung some swimmers … swim at your own risk.”

Pete Gawne of the New England Aquarium told the Globe, “One of the questions we get a lot is how to predict this [situation], but there’s just so many inputs, from the water temperature to prevailing currents and wind that put these into contact with people.”

Contrary to myth, experts don’t advise treating a jellyfish sting with urine. Instead, pick any tentacle fragments out with tweezers, soak the affected area in hot water, and apply hydrocortisone ointment twice daily, the Mayo Clinic recommends.