A dark, silvery substance that forced dozens of Indiana beach swimmers out of Lake Michigan has mysteriously vanished. The unidentified dark layer spanned a two-mile area of water.

"Some people described it as looking like silvery or almost metallic. And it was sticking to algae and people in the water," Bruce Rowe, a ranger from the National Park Service, told ABC's Chicago station WLS-TV.

On Monday, swimmers at a Northwest Indiana beach came out of the water with an “oily substance” on their bodies, according to the Coast Guard and the Indiana Department of Environmental Management. Preliminary tests shows that the slimy substance contained D-gluconic acid, a mild acid used to clean metals, and tricalcium orthophosphate, an additive found in food and fertilizers, said Dan Goldblatt, a spokesman for the department.

Tricalcium orthophosphate -- an anti-caking agent -- is manufactured by a plant in the lakeside town of Porter, Ind., that ships the material out on barges, but it is not suspected as the source of the slimy material, IDEM officials told the Times of Northwest Indiana.

On Wednesday, the substance disappeared. "They checked the beach, and they can't find any evidence of it," IDEM spokesman Barry Sneed told ABCNews.com. "[Authorities] figure it may have sunk, or moved farther north. It's a strange phenomenon."

In the past, Lake Michigan has seen advisories for E. coli, Rowe said. But this substance is something new. “I've never heard of happening in our park, at least," he said.

The Coast Guard says the substance is “not petroleum-based” and has been reviewing video footage from the past week of vessels in the Port of Indiana, but have not seen any signs of spills, Petty Officer 3rd Class Christopher Yaw told the Chicago Tribune.

Indiana officials plan to keep swimmers off the beaches until the silvery film is identified. "Until we find out otherwise, we're going to err on the side of caution and keep the beach advisory in effect," Rowe said.

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