Drive on I-94 just outside Chicago between Gary and Michigan City, Ind., and you catch a glimpse of the massive sand dunes that make up the Indiana Dunes National Lakeshore. Tucked between coal plants and steel mills, the dunes are as high as 200 feet, stretching along the southernmost tip of Lake Michigan.

These dunes were formed some 14,000 years ago, and most are now covered in forest, says park ranger Bruce Rowe, who has worked at the lake shore for 30 years.

"Most of the dunes at Indiana Dunes, people don't recognize as dunes, because they're hiding under an oak forest," Rowe says.

But one dune, Mount Baldy, is different. It's a big pile of sand with not a blade of beach grass to speak of, just a couple of scruffy trees struggling to survive.

Rowe says that's partly because of the dune's popularity. About 200,000 people visit the park each year, almost all to get their feet on Mount Baldy.

"For generations, people climbed up and down Mount Baldy, had a wonderful time," Rowe says. "Along the way, [they] may have helped damage it in a small way as well."

But the park didn't take the damage seriously until last July, when 6-year-old Nathan Woessner of Sterling, Ill., was investigating a small depression in the sand in an area that was blocked off for restoration.

The depression suddenly gave way, swallowing him deep into the sand. Rescuers had to dig down 11 feet into the sand to get Woessner out after a three-hour burial. Rowe says Nathan was injured but has now fully recovered.

"We certainly don't want to blame a 6-year-old for seeing a hole just outside the official area and wanting to investigate it," he says. "That's one of the reasons why the entire dune in the area is now closed, so that something like that doesn't happen again."

Erin Argyilan, professor of geosciences at Indiana University Northwest, was doing a wind study on Mount Baldy the day the hole opened up. She says at the time, she could not believe what was happening. She told the Washington Post that she thought Nathan's parents, who were frantically screaming and pointing to the spot the boy disappeared, were mistaken.

Holes that narrow and deep enough to swallow a small child don't just open up in the sand. It contradicted everything she knew about how dunes are structured.

"I called every geologist who would talk to me and the response was pretty much the same: That can't happen. Never heard of that happening. It cannot happen. Something else must have happened," Argyilan says.

But what is happening? Since last July, three more holes have formed and collapsed within the dune. Scientists still have no idea why.

"In most cases, what we can do is we can walk up and we can see the structure of a hole," she says. "It's been less than a foot across in diameter. And they seem to have a pretty solid structure to them but they fill in rapidly."

Argyilan says their best guess is that the holes are caused by decomposing trees that have been buried under the sand dune for nearly a century.

"The age of the materials and the wet conditions during the spring of 2013 may have forced these materials to become unstable, collapsing and creating openings to the surface," explains the park's website.

She recently spoke with Oregon residents, who told her they've seen same thing happen in a system of sand dunes on the Pacific Coast. That it may be happening in other dunes is an interesting development.

Mount Baldy is closed for the summer while Argyilan and National Park Service scientists use ground-penetrating radar and GPS devices to peer beneath the surface, trying to solve the mystery. The Park Service will also plant new grass where it grew until the last century to try to stabilize the dune and prevent new holes.

Meanwhile, the rest of the park's hiking trails, beaches, forests and wetlands spanning 15 miles of the southern shore of Lake Michigan are open.

"The sort of misunderstanding that's been out there is that the ground suddenly opens up and swallows people," Argyilan says. "It definitely is not a man-eating dune."

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