Jenny Greenleaf was glad that it was hot enough to go barefoot when she and her husband took regular walks on York Beach in southern Maine this week.
But when they returned to their beach chairs and began to remove the sand from their feet, they noticed that their normally yellow soles were a deep black.
“I’ve never seen anything like this,” said book designer and artist Ms. Greenleaf. “It was almost like I walked through charcoal.”
At home, he took a bath and cleaned his feet, but the stain which was neither greasy nor greasy, it would come out only partially.
Along the southern coast of Maine, as well as in neighboring New Hampshire, many others were also struggling to remove dark spots from their feet.
“I still can’t get it off,” said Kyra O’Donnell, who has black legs, as she visited Great Island Common Beach in New Hampshire, about 14 miles south of York Beach, on Sunday.
Robin Cougar, director of Parks and Recreation in York City, said she had about 100 calls and emails this week about stained feet. Similar news on social media also came from South Gloucester, Mass., and as far north as Wales, Maine, spanning more than 70 miles.
Theories were plentiful. Algae and oil were common. “In Hawaii, volcanic gas may have turned sand black, but there are no volcanoes in Maine, so it’s probably something gross,” wrote one person in a local Facebook group.
Ms Greenleaf had a fringe theory, which was also ridiculed by her own husband, involving a submarine she had seen in the area.
“Maybe the submarine blew away the cloud of insolence,” she said.
On Wednesday, Jim Britt, a spokesman for Maine’s Department of Agriculture, Conservation and Forestry, offered a possible answer: Millions of tiny black kelp flies that feed on rotting seaweed died on a stretch of beach.
“Don’t know why,” he said. “Nature does crazy stuff. This could be one of those examples.”
He said the carcasses of the insect, which appeared to have been washed with ashes, contained a naturally occurring pigment.
Efforts were underway to identify the specific type of kelp fly, which would also help to know where they came from. Despite this, stepping on them does not pose any health problems, Mr Britt said.
He could not answer whether it would be bad for dogs to eat them, a question that some people were asking on social media.
Linda Stathoplos, a retired National Oceanic and Atmospheric Administration oceanographer, conducted her informal examination of sand at Wells Beach in southern Maine on Tuesday, collecting a sample and observing it under a microscope.
“There were tons and tons of tiny insects about the size of the tip of a pin,” she said. Some had two wings. Others had four. “They were all dead, of course.”
She could never remember a similar mass “death event” of flies.
Neither could Joseph Kelly, a marine geologist at the University of Maine. “I’ve worked on the geology of beaches on the Maine Coast for 40 years and have never seen anything like it (or heard about it anywhere else),” he said in an email.
On Facebook, beach walkers with stained feet listed all the ways they’ve tried to remove the stains. Neither dish soap nor baby wipes were particularly successful.
Ms Greenleaf later discovered a solution for cleaning her feet while walking unintentionally. She and her husband had returned to the beach after the rain to find clues. Although the search was fruitless, by the end of their walk the sand and stones had removed the stains.
“Our feet were spotless,” she said.