(CNN) -- For 3 1/2 hours, rescuers dug desperately to reach the 6-year-old boy swallowed whole in the northern Indiana sand dunes.
But the more they dug, the deeper Nathan Woessner seemed to sink in the 11-foot mound of 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," one of the rescuers, Rich Elm, told CNN affiliate WNDU.
"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."
Six-year-old Nathan Woessner and his family had gone to Mount Baldy Beach at the Indiana Dunes National Park on Friday. It was a day meant to be a fun family vacation at the beach on the Lake Michigan shoreline.
But then the unexpected happened. Every parent's worst nightmare: sounds of screams and a missing boy.
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.
Relatives and loved ones scrambled to dig through the sand. It didn't help.
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 excavation equipment and shovels in hand. They raced against the clock.
An hour went by.
"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 trapped vertically in the sand.
"Once I had a hold of his head, I was supporting to him and just talking to him you know, just like I would talk to my own son," Kreighbaum said.
An unconscious Nathan was rushed to a hospital. Authorities do not believe he suffered brain damage from lack of oxygen, or any other life threatening injuries.
A single air pocket in the dune may have saved his life.
"When he arrived, he was able to respond to simple commands," University of Chicago Medicine Comer Children's Hospital said in a statement.
The boy remained in critical condition late Sunday night.
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. Its half-buried trees are proof.
"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 CNN affiliate WNDU. "It's baffling."
The beach will remain closed at least until Monday as authorities investigate what caused the sand to give way.
CNN's Jennifer Moore contributed to this report.