San Diego residents have reported sightings of a rare black jellyfish known as sea nettles at local beaches.



The dark purple sea creatures were last seen about a year ago, and have now returned to local waters – sometimes delivering a painful sting to anyone that bothers them. And they’re not small – black jellyfish can grow up to three feet wide, with tentacles spanning 10 feet long.



A group of children at Pacific Beach were stung on Sunday afternoon, and said they witnessed others getting hurt as well.



But the sting isn’t too bad – the kids said it wasn’t painful for long.



“It lasts about 20 minutes,” said Tyler Crabtree.



It’s not certain why the jellyfish come to San Diego’s beaches, but scientists speculate it’s because they’re attracted to waters with low oxygen levels. This year marks the fifth time in a little more than a decade since they’ve been seen.



Earlier this month, black jellyfish were seen at Laguna Beach. People were got stung there said it felt like a bee sting.



Anyone who suffers a sting should wash it out with water, and should avoid rubbing the area so toxins do not spread.