Officials believe the substance is a common food additive. Work continues to figure out where the substance came from.
Warnings posted at the beach were not enough to keep some from venturing into the water. But most beachgoers were staying out of the surf.
"I'm on vacation, so I have to make the most of it. Hotel's already paid for so might as well enjoy the beach the best way we can," said Jeanie Renforth.
The swim warnings were up along a ten-mile stretch of beach from the town of Porter to Michigan City because of a quarter-mile slick of a mysterious substance found Monday in the water.
"Some people described it as looking like silvery or almost metallic. And it was sticking to algae and people in the water," said Bruce Rowe, National Park Service.
"Kind of disappointed because we drove about three hours to get here. We didn't know there was going to be a substance or anything," said Alexander Myers.
State environmental officials say so far no one got sick from the substance, which preliminary tests show to be tricalcium orthophosphate, a common anti-caking additive used in food.
The Coast Guard is now investigating where it came from, perhaps a nearby factory or vessel carrying materials.
"This is very unusual. We do sometimes have advisories for E. coli in the water. But something like this I've never heard of happening in our park, at least," Rowe said.
State officials say they won't know for sure what was in the water, and in what concentration, until final test results come back later this week. Though Tuesday's high waves have helped disperse the substance.
"Fortunately, it's kind of a cool day. And so there probably wasn't going to be a lot of people coming to the beach. So I think for the economy of the area, it wasn't a massive loss," said Nicole Messacar.
"They told us that we just couldn't swim in the water. We could along the edge of the beach and get our feet wet, but that's about it," Renforth said.
"Something's in the water and it shouldn't be there," said Carl Dahlin, a Porter, Ind., resident.
Dahlin says he's never seen anything like it here at the Indiana Dunes. He says he saw several people emerge 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.
The Times of Northwest Indiana reports that an unidentified Porter plant manufactures the material and ships it out of the Port of Indiana on barges.