Jenny Greenleaf was thrilled that it was finally warm enough to go barefoot when she and her husband took regular walks along York Beach in southern Maine this week.
But when they got back to their beach chairs and started brushing the sand off their feet, they noticed that their normally pale soles were a deep black.
“I’ve never seen anything like it,” said Ms Greenleaf, book designer and artist. “It was almost like I was walking through charcoal.”
At home, they showered and rubbed their feet, but the stain, which was neither slippery nor greasy, only partially disappeared.
Along the southern 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 seem to take it off,” said Kyra O’Donnell, who has had black feet since visiting the beach at Great Island Common in New Hampshire, about 14 miles south of the York Beach, Sunday.
Robin Cogger, director of parks and recreation for the City of York, said she responded to around 100 calls and emails this week about stained feet. Similar reports on social media have come as far as Gloucester, Mass., And as far north as Wells, Maine, a range of over 70 miles.
Theories abounded. Seaweed and oil were common. “In Hawaii, the sand can turn black from volcanic gas, but no volcanoes in Maine, so it’s probably something gross,” a man wrote in a local Facebook group.
Ms Greenleaf had a fringe theory, which even her own husband scoffed at, which involved a submarine she had seen in the area.
“Maybe this submarine belched out a cloud of nastiness,” she said.
Maine Department of Agriculture, Conservation and Forestry spokesperson Jim Britt on Wednesday offered the likely answer: Millions of tiny black kelp flies that feed on decaying algae appeared to have died on part of the beach.
“We don’t know why,” he said. “Nature does crazy things. It could be one of those cases.
The insect carcasses, which appeared to have washed up on the shore, contain a natural pigment, he said.
Efforts were underway to identify the particular type of kelp fly, which should also help determine their origin. Either way, walking on them is not a health problem, Mr Britt said.
He couldn’t answer if it would be bad for dogs to eat them, a question some were asking on social media.
Linda Stathoplos, a retired National Oceanic and Atmospheric Administration oceanographer, conducted her own informal examination of the sand in Wells Beach, southern Maine on Tuesday, taking a sample and examining it under a microscope.
“There were tons and tons of little bugs, the size of a pinpoint,” she said. Some had two wings. Others had four. “They were definitely all dead.”
She could not recall ever hearing of a similar mass “death event” of flies.
Nor Joseph Kelley, a marine geologist at the University of Maine. “I have worked on beach geology for 40 years on the coast of Maine and have never seen anything like it (or heard of anywhere else),” he said in an email.
On Facebook, beach vacationers with soiled feet listed all the ways they had tried to remove the stain. Neither dish soap nor baby wipes were particularly successful.
Ms Greenleaf inadvertently discovered a foot cleaning solution on a subsequent walk. She and her husband had returned to the beach after a rain to look for clues. Although the search was unsuccessful, by the end of their walk the sand and stones had polished the spots.
“Our feet were immaculate,” she said.