Jenny Greenleaf was glad it was finally warm enough to walk barefoot when she and her husband took their regular walk along York Beach in Southern Maine this week.
But when they returned to their beach chairs and began brushing the sand off their feet, they found their normally pale soles were a deep 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 wasn't slippery or greasy, only partially came out.
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 still can't get it off," said Kyra O'Donnell, who has black feet since she visited New Hampshire's Great Island Common beach, about 14 miles south of York Beach, on Sunday.
Robin Cogger, the director of Parks & Recreation in the city of York, said she had received about 100 calls and emails this week about stained feet. Similar social media posts came from the south as Gloucester, Mass., and as far north as Wells, Maine, a span of over 70 miles.
Theories abounded. Algae and oil were common. "In Hawaii, the sand can be blackened by volcanic gas, but not volcanoes in Maine, so it's probably something dirty," one man wrote in a local Facebook group.
Mrs. Greenleaf had a fringe theory, which even her own husband scoffed at, that involved a submarine she had seen in the area.
"Maybe that submarine threw up a cloud of misery," she said.
On Wednesday, Jim Britt, a spokesman for the Maine's Department of Agriculture, Conservation and Forestry, offered the likely answer: Millions of tiny black kelp flies feeding on rotting seaweed appeared to have died on a stretch of beach.
"It is not known why," he said. “Nature does crazy things. This could be one of those cases."
The insect carcasses, which appeared to have washed ashore, contained a naturally occurring pigment, he said.
Efforts were made to identify the specific species of kelp flies, which should also help determine where they came from. Either way, stepping on it poses no health problems, Britt said.
He couldn't answer whether it would be bad for dogs to eat them, a question some asked on social media.
Linda Stathspreis, a retired oceanographer with the National Oceanic and Atmospheric Administration, did her own informal survey of the sand at Wells Beach in southern Maine on Tuesday, collecting a sample and examining it under a microscope.
"There were tons and tons of little critters, about the size of a pin," she said. Some had two wings. Others had four. "They were certainly all dead."
She couldn't remember ever hearing about a similar massive 'death event' of flying.
Neither is Joseph Kelley, a marine geologist at the University of Maine. "I've spent 40 years working on the geology of beaches on the coast of Maine and never seen (or heard about) anything like it," he said in an email.
On Facebook, beach vacationers with sore feet posted all the ways they'd tried to remove the stain. Neither dish soap nor baby wipes were particularly successful.
Mrs. Greenleaf accidentally discovered a foot cleansing solution on her next walk. She and her husband had returned to the beach after a rainstorm to look for clues. Though the search was fruitless, by the end of their walk the sand and stones had buffed away the stains.
"Our feet were spotless," she said.