Millions of mayflies swarm lakeside communities each summer. 48 hours later they die, leaving behind a slippery mess that can make roads dangerous.

• Millions of mayflies swarm lakeside communities every year and land on any surface they can find.
• Mayfly corpses decompose into a fatty, slippery mess that can make roads and sidewalks dangerous.
• Popular TikToks have documented the insects crawling over people, gas stations, and bridges.

A dense, brown haze rose up in the field next to Tommy Springer.

Nearly silent, the cloud wasn't smoke or mist - it was thousands of mayflies fluttering in a thick swarm as they mated at sunset.

"Viewing the swarm was like looking through an amber window screen or tan mesh that had been suspended above the field," Springer, a wildlife and education specialist in Fairfield County, Ohio, told Insider. "I could still see the trees and landscape beyond it, it was just distorted by a cloud of thousands of insects holding a stationary position en masse."

Sights like this are common in lakeside communities across the US. For a few days each summer, mayflies break through the surface of lakes and rivers to begin a mating frenzy. For the few days they live above water, they fly around in swarms and land on any available surface, including people, cars, and buildings.

"They're harmless - they can't sting, they can't bite. They're just a mess," Dave McShaffrey, a biology professor at Marietta College, told Insider.

Mayfly corpses often pile up on roads and sidewalks, where they break down into a fatty, slippery glaze that can cause wipeouts for pedestrians and car accidents for drivers. They also stink like rotten fish when their bodies decompose and are squashed by feet and tires, Springer said.

"When their bodies mix with the water they form an almost muddy, slick, slushy feeling. It's like a slushy winter snowstorm," Springer said.

The insects are found all over the country, but the biggest swarms tend to appear along the Great Lakes and the Mississippi River. Port Clinton, Ohio, a city located next to Lake Erie, is famous for its yearly mayfly outbreaks.

Though the insects often congregate near gas stations and other stores that keep their lights on at night, George Black, an employee at a Sunoco Gas Station in Port Clinton, said the mayflies don't drive customers away. Instead, customers show up early to the gas station to collect the deep piles of corpses that have become a hot commodity for local residents.

"People shovel them up and use them as fertilizer in their yards and gardens," he said. "People love them. They're almost fighting over them."

Municipalities have different ways of dealing with the annual emergence. In June, Pennsylvania officials shut down a bridge at night as the throng of mayflies that congregated by the bridge's lights made driving too dangerous. Port Clinton typically turns off lights along one of the main thoroughfares next to Lake Erie and dispatches teams with leaf blowers to clear the insects off of sidewalks and public property in the morning, Tracy Colston, safety service director for Port Clinton, said.

"We got pelted pretty good with them," he told Insider.

Mayfly larvae burrow into the silt of lakes and large rivers where they live for about a year until the water reaches a certain temperature every summer. Then, millions of the insects, with dragonfly-like wings, bulging eyes and hair-like tails, rise to the surface and take flight, McShaffrey said.

From there, they fly with the wind to the adjacent shore and flock together for mating flights. The mayflies die soon after the females drop their eggs back into the water, McShaffrey said.

Though the insects are a nuisance, they're a good source of food for fish and swarms of them are an indication of clean water, as mayflies have a a very low toleration for pollution.

If they bother you, simply turn off the lights.

"They're going to die in 48 hours anyways," McShaffrey said. "You just sit back and try to enjoy them a couple days out of the year."