Jenny Greenleaf was delighted that it was finally warm enough to go barefoot as she and her husband took their regular walk on York Beach in south Maine this week.
But when they returned to their beach chairs and started wiping the sand off their feet, they noticed that their normally pale soles were jet black.
“I’ve never seen anything like it,” said Ms. Greenleaf, a book designer and artist. “It was almost like walking through charcoal.”
At home they showered and scrubbed their feet, but the stain, which was neither slippery nor greasy, only partially peeled off.
Along the south coast of Maine, as well as in neighboring New Hampshire, many others also struggled to remove dark spots from their feet.
“I’m still not getting it,” said Kyra O’Donnell, who has had black feet since visiting Great Island Common in New Hampshire, about 14 miles south of York Beach.
Robin Cogger, the director of Parks & Recreation in the City of York, said she received about 100 calls and emails this week about discolored feet. Similar reports on social media came from the south like Gloucester, Massachusetts, and as far north as Wells, Maine, a span of more than 70 miles.
Theories abounded. Seaweed and oil were common. “In Hawaii, volcanic gas can black the sand, but Maine doesn’t have volcanoes, so it’s probably something gross,” wrote a man on a local Facebook group.
Ms. Greenleaf had a marginal theory that even her own husband mocked about a submarine she had seen in the area.
“Maybe the submarine spat out a cloud of meanness,” she said.
On Wednesday, Jim Britt, a spokesman for the Maine Department of Agriculture, Conservation and Forestry, gave the likely answer: Millions of tiny black kelp flies that feed on decaying algae appear to have died on a stretch of beach.
“It is not known why,” he said. “Nature does crazy things. This could be one of these cases. “
The insect carcasses that appear to have washed ashore contain a naturally occurring pigment, he said.
Efforts have been made to identify the particular species of kelp fly, which should also help clarify its origin. Regardless, stepping on them doesn’t pose any health problems, Britt said.
He couldn’t answer whether it would be bad for dogs to eat them, a question some people asked on social media.
Linda Stathoplos, a retired oceanographer with the National Oceanic and Atmospheric Administration, conducted her own informal survey of the sand at Wells Beach in southern Maine on Tuesday, taking a sample and viewing it under a microscope.
“There were tons and tons of little bugs about the size of the tip of a pin,” she said. Some had two wings. Others had four. “They were definitely all dead.”
She couldn’t remember ever hearing of a similar mass fly mortality event.
Neither could Joseph Kelley, a marine geologist at the University of Maine. “I’ve worked on the geology of beaches on the Maine coast for 40 years and I’ve never seen (or heard of) anything like it before,” he said in an email.
On Facebook, beach goers with rotten feet listed all the ways they had tried to remove the stain. Neither dish soap nor baby wipes were particularly successful.
While taking a walk afterward, Ms. Greenleaf accidentally discovered a foot cleansing solution. She and her husband had returned to the beach after a rain to look for clues. Although the search was in vain, the sand and stones by the end of their walk had polished away the stains.
“Our feet were spotless,” she said.