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The Indiana Dunes National Lakeshore is a kind of urban oasis. It's not far from Chicago, amid the industrial cities of northern Indiana. It's a busy summer spot but one of its most popular areas is now closed indefinitely. That's because scientists are trying to unravel a mystery. They want to know why dangerous sinkholes have been opening up in the dunes. Michigan Radio's Lindsey Smith reports.

LINDSEY SMITH, BYLINE: Drive on I-94, just outside of Chicago, between Gary and Michigan City, Indiana, and you can 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 southern-most tip of Lake Michigan. Park ranger Bruce Rowe started working at the national park 30 years ago.

BRUCE ROWE: Most of the dunes at Indiana Dunes people don't recognize as dunes because they're hiding under an oak forest.

SMITH: Rowe says the dunes were formed some 14,000 years ago, so most of them are now covered in forest. But one dune at the park is different. Mount Baldy is, well, it's bald. It has no beach grass to speak of, just a couple scruffy looking trees struggling to survive. Mostly it's just a big pile of beach sand.

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

ROWE: So, yeah, for generations people have climbed up and down Mount Baldy, had a wonderful time, along the way may have helped damage it in a small way, as well.

SMITH: But the park didn't take the damage too seriously until last July. That's when six-year-old Nathan Woessner 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. Rowe says he was injured but has now fully recovered.

ROWE: We certainly don't want to blame a six-year-old boy for seeing a hole just outside the official area and wanting to investigate it. 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: The sort of misunderstanding that's been out there is that the ground suddenly opens up and swallows people. It definitely is not a man-eating dune.

SMITH: That's Erin Argyilan. She's a professor of geosciences at Indiana University Northwest.

ARGYILAN: In most cases, what we can do is we can walk up and we can see the structure of a hole. And 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.

SMITH: Argyilan was doing a wind study on Mount Baldy the day the hole opened up. She says at the time, she really could not believe what was happening. Holes that narrow and deep enough to swallow a small child don't just open up like that in the sand. It contradicted everything she knew about how dunes are structured.

ARGYILAN: I called every geologist that would talk to me and the response was pretty much the same; that can't happen, we've never heard of that happening, it cannot happen, something else must have happened.

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

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

Erin Argyilan is joining with other scientists this summer to try to figure out just what's going on with these dunes.

For NPR News, I'm Lindsey Smith.

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