MICHIGAN CITY, Ind. (CBS) — Park rangers have brought in ground-penetrating radar to help determine why a 6-year-old boy plunged 11 feet under the sands of Mount Baldy at the Indiana Dunes a week ago.

WBBM Newsradio’s John Cody reports Nathan Woessner was improving at the University of Chicago’s Comer Children’s Hospital on Thursday, his condition upgraded from critical to serious, and his breathing tube removed.

He fell into a hole as he and his father were climbing Mount Baldy, and he was buried 11 feet below the surface for nearly four hours before rescuers could get him out. He was trapped in an air pocket that kept him from suffocating.

Nathan Woessner was taken to Comer Children's Hospital after he fell into and was buried a a sand hole at the Indiana Dunes. (Credit: Woessner Family Photo)

Nathan Woessner was taken to Comer Children’s Hospital after he fell into and was buried a a sand hole at the Indiana Dunes. (Credit: Woessner Family Photo)

Park Ranger Bruce Rowe said the incident remains a mystery.

“They’ve been reaching out to geologists – both within the National Park Service, and outside at universities – and so far, nobody that we’ve found has ever heard of such a phenomenon,” Rowe said. “With the experts that we’ve spoken to first – and they’re expert geologists on coastal dunes in the Great Lakes – they have not heard of this, and certainly our experience at the national lakeshore has not seen this sort of phenomenon before.”

Park officials have theorized a rotting tree buried under the sand might have left a void under the sand, but have yet to confirm exactly how Nathan was able to survive under the sand for so long.

Rowe said investigators are taking a two-step approach to the mystery.

First, investigators will measure electrical conductivity of all 43 acres of the Mount Baldy area, to see if there are any unusual findings.

“What we’ll then follow-up is to look at each of those anomalies in a lot more detailed fashion, by getting a ground-penetrating radar unit,” he said.

That will allow them to see if any process – natural or man-made – has created any other voids in the sand.

Rowe, spokesman for the Indiana Dunes National Lakeshore, said the area remains roped off, off-limits for visitors until everyone’s safety can be assured.

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