Story highlights Nathan Woessner, 6, was visiting the northern Indiana sand dunes with family

The sand dunes, which line Lake Michigan, can sometimes give way

Nathan is in critical condition but does not appear to have any brain damage

The 6-year-old boy swallowed whole in an Indiana sand dune over the weekend could make a full recovery, a medical official said Monday.

Rescuers dug furiously for three and a half hours to extract little Nathan Woessner, who was recovered unconscious and rushed to the University of Chicago Medicine's Comer Children's Hospital.

But Tracy Koogler, medical director at the hospital's pediatric intensive care unit, said doctors are signaling progress and possibly a full recovery.

The boy is on a ventilator, she said, and is breathing at the capacity of a moderate intensive-care-unit pneumonia patient.

He could have his breathing tube out later this week and could be out of the hospital in 10 days to two weeks with rehabilitation to follow, Koogler said. It's unclear what kind of lasting health issues he may have.

Authorities have said they don't believe Nathan suffered brain damage from a lack of oxygen and there were no significant injuries to his eyes. A single air pocket in the dune may have saved his life, officials said.

At present, doctors are trying to clear sand from his lungs.

A family outing turned nightmare

The boy and his family had gone Friday to Mount Baldy Beach at Indiana Dunes National Park, in the northwestern corner of the state. It was a day meant to be a fun family vacation at the beach on the Lake Michigan shoreline.

Pastor Don Reul, Nathan's grandfather, said his daughter and son-in-law went with another couple and their children. The incident occurred after the fathers decided to climb Mount Baldy. Nathan and a friend followed.

Then the unexpected happened: sounds of screams and a boy lost in the sand.

Nathan had stepped in a sinkhole, and a friend yelled to the fathers that the child was gone, the grandfather said.

The fathers frantically tried to dig him out, but the more they dug, the deeper Nathan seemed to sink in the 11-foot mound.

A call for help went out immediately.

Rescuers stay focused

911 Operator: 9-1-1.

Caller: I'm at the Mount Baldy Beach. And my friend's son, he got stuck in the sand dune, and he's like under the sand and they can't get him out.

911 Operator: OK, can anybody see him or is he completely covered by sand?

Caller: Uh, yes. My husband and his dad are trying to dig him out.

Dozens of first responders rushed to the scene with shovels in hand. Heavy excavation equipment also was called in. They raced against the clock.

"There was lots and lots of guys hand digging, trying to expose him making sure nobody was going to hurt him or anything with any equipment," one of the rescuers, Rich Elm, told CNN affiliate WNDU

An hour went by.

Then another.

"We were really losing hope fast, and we tried to just stay focused," Michigan City firefighter Brad Kreighbaum told CNN affiliate WSBT . "The first two hours was complete misery."

More than three and a half hours later, signs of life.

Nathan was cold and appeared lifeless but had a heartbeat. He was trapped vertically in the sand.

"One minute you're thinking, 'We don't know what we're going to have,' and you're thinking the worst. Then you're hoping for the best," Elm said.

"Once I had a hold of his head," Kreighbaum said, "I was ... just talking to him, you know, just like I would talk to my own son."

'Never heard of anything like this'

Park rangers do not know what caused the hole. Mount Baldy is the tallest moving sand dune in the national lakeshore, according to the National Park Service. Half-buried trees show its shifting nature.

"I've been a park ranger here at Indiana Dunes National Lakeshore since 1991, and I've never heard of anything like this here or at other sand dune parks," Park Ranger Bruce Rowe told WNDU. "It's baffling."

The beach was closed Monday as authorities investigated what caused the sand to give way.