Investigators are trying to identify a substance that showed up in the water Monday.
Preliminary tests show "metallic flakes" and authorities most of it is harmless, but it's not clear yet what else is in the substance.
Chopper 7 HD was overhead as a dark, slimy substance snakes its way through the southern tip of Lake Michigan.
"Something's in the water and it shouldn't be there," said Carl Dahlin, a Porter, Indiana resident.
Dahlin says he's never seen anything like it here at the Indiana Dunes. Several people emerged from the water coated in the stuff.
"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. It became worrisome at that point," said Dahlin.
Authorities quickly closed the beach to swimming. An early analysis only concluded it's not oil.
As a precaution: Beaches from Porter to Michigan City, Indiana will be closed to swimming until they determine what is floating in the lake.
"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," said Gene Davis, Indiana conservation officer.
Preliminary tests indicate it is a chemical used in food preservation, but another round of tests is scheduled for Tuesday morning. Indiana conservation officers are asking people to stay out of the water until they can determine what the substance is.