People living on Miami Beach near Collins Avenue and 64th Street started seeing mysterious dark trails in the ocean over the weekend.
One resident, Chris Mayor, snapped photos that seemed to show a murky slick in the sea.
PHOTOS: Dark trails in water
Dark trails could be seen along the coast and spreading out into the deep.
"It's weird. What's going on?" asked one beachgoer.
Some said they feared it was an oil spill.
But, the Army Corps of Engineers and environmental experts with Miami-Dade County said it isn't an oil slick. It's rust.
Brian Flynn, an environmental expert with the county, said the rust started making an appearance when crews began dredging last week for a beach renourishment project.
"The first thing they do is they have to lay 8,000 feet of pipe going from the shoreline out to where the dredge will actually be pumping," Flynn said.
He said rusting is normal.
"The rust problem that cropped up is essentially because the pipe has a coating on the inside because it's almost always underwater. And when they lay it down underwater, all that rust liquefies, and it comes out," he said.
On Wednesday, some swimmers said they worried about safety. Flynn said the rust does not pose any environmental or health hazards.
"There's no problem with that. In, fact it's very -- it looks worse than it is," he said.