Officials remain baffled as to how a dark, slick substance that forced dozens of swimmers out of the water at a northwest Indiana beach mysteriously vanished.
"They checked the beach, and they can't find any evidence of it," Indiana Department of Environmental Management spokesman Barry Sneed told ABCNews.com. "[Authorities] figure it may have sunk, or moved farther north. It's a strange phenomenon."
Swimmers notified law enforcement authorities that a dark-colored residue stretching nearly a mile long on Lake Michigan had appeared on the surface of the water at Porter Beach in Porter, Ind., Monday afternoon, Sneed said.
Porter Fire Department Deputy Chief Jay Craig told ABCNews.com that when he arrived at the lake the water looked slick with what appeared to be oil. Upon further inspection, the substance was a gun-metal gray with metal flakes in it.
Craig said you could tell how deep someone had been in the water depending on where their bodies were stained with the dark residue.
"They were worried when they saw two kids come out of the water and the one was, his head and half his body was covered a bit in black," Porter resident Carl Dahlin told ABC's Chicago station WLS-TV.
"We are completely baffled as to what it truly is, whether it came from one of the steel mills or something out of the smokestacks or possibly washed off one of the big barges as they came into the harbor," Gene Davis, Indiana conservation officer, told WLS-TV.
Officials shut down the beach as the Coast Guard, the Indiana Department of Natural Resources and the National Park Service were called in to help identify the slime.
The appearance of the unidentifiable slick forced the closure of five northern Indiana beaches on Lake Michigan, Sneed said.
"In my eight years here, we've never had to close the water because of an unknown substance like this," said Indiana Dunes State Park Manager Brandt Baughman. "We're not going to open our beach until we know what it is."
Indiana Dunes State Park remains closed as a result of strong waves and rip tides, but other beaches in the area have issued a swim at your own risk advisory.
Meanwhile, officials work to determine what the mysterious substance was and where it could have gone.
While authorities worried that the sheen could spread east toward Michigan City, Ind., Sneed said the substance was nowhere to be found this morning.
Sneed said that while preliminary testing of water samples indicated the mystery sheen might have been a food additive that was also used in fertilizer, this morning's reading revealed it might have been a type of acid.
While the tests yield variable results, samples were sent to a lab for further analysis, Sneed said.