Beach-goers looking to feel the sand between their toes have instead ended up scrubbing away the black stain from millions of dead bugs.
The tiny carcasses littered Wells Beach on the coast of the US state of Maine.
A Maine Geological Survey expert said he hadn't seen anything like it in 35 years. The bugs haven't been identified but some reports say it's a kelp fly.
The otherwise harmless bugs appear to have rolled in on the tide and most now seem to have rolled out the same way.
Other beaches in the area were affected too.
One beachgoer, Ed Smith, told the Portland Press Herald he'd noticed the bugs earlier in the week and others had too.
"I sat on the edge of my tub with [dishwashing soap] and a scrub pad, and I still couldn't remove the stain from my feet," he told the Press Herald.
The Maine DEP now says pigment from flying insects washing up on shore is why some beachgoers’ feet have been turning black: https://t.co/k3SNdmNDKm— CBS 13 News (@WGME) June 10, 2021
One retired oceanographer, Linda Stathoplos, put the material under her microscope and concluded: "It was clearly little bugs."
Maine Geological Survey (MGS) marine expert Steve Dickson was called in.
"Normally this time of year we get calls about too much seaweed (wrack) on the beach and the swarming flies that hang around the decaying seaweed. This wasn't that," he said.
But he reckons the bugs will soon disappear back where they came from. And the stain? Maybe from pigments in plants the bugs ate.
Kelp flies do normally feed on stranded and rotting seaweed.
Other beachgoers posted images of blackened feet but some were unclear whether it was the bugs or more traditional beach tar.
Local resident Kate Phelps told WGME-TV: "I've heard of tar feet before, so that's what I assumed it was. We noticed it when we were here and we just figured it would come off in the shower. We were pretty surprised that it didn't come off."
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