PICTURES, VIDEO

US boy buried in sand dune sinkhole in critical condition

In this July 12, 2013 file photo, Michigan City police and firefighters dig with shovels to rescue Nathan Woessner, of Sterling, Illinois near Michigan City, Indianapolis. -- FILE PHOTO: AP
In this July 12, 2013 file photo, Michigan City police and firefighters dig with shovels to rescue Nathan Woessner, of Sterling, Illinois near Michigan City, Indianapolis. -- FILE PHOTO: AP
In this Friday, June 12, 2013, photo, Michigan City firefighters, police, and first responders dig through a sand dune at Mount Baldy near Michigan City, Indianapolis. -- FILE PHOTO: AP
In this Friday, June 12, 2013, photo, Michigan City firefighters, police, and first responders dig through a sand dune at Mount Baldy near Michigan City, Indianapolis. -- FILE PHOTO: AP
In this Friday, July 12, 2013, photo, family members of an 8-year-old boy who was buried in the sand at Mount Baldy are escorted to the beach by a Michigan City Police Department chaplain in Michigan City, Indianapolis. -- FILE PHOTO: AP
In this Friday, July 12, 2013, photo, family members of an 8-year-old boy who was buried in the sand at Mount Baldy are escorted to the beach by a Michigan City Police Department chaplain in Michigan City, Indianapolis. -- FILE PHOTO: AP
In this July 12, 2013, photo rescue workers with a stretcher carrying 6-year-old Nathan Woessner after he was pulled from a sand dune at the Indiana Dunes National Lakeshore in Michigan City, Indianapolis. Doctors said on Monday, July 15, that the bo
In this July 12, 2013, photo rescue workers with a stretcher carrying 6-year-old Nathan Woessner after he was pulled from a sand dune at the Indiana Dunes National Lakeshore in Michigan City, Indianapolis. Doctors said on Monday, July 15, that the boy, who was buried for hours the sand dune, is responsive and expected to make a full neurological recovery. -- FILE PHOTO: AP
This undated photo provided by the Woessner family via The University of Chicago Hospital, shows Nathan Woessner. The doctor who helped treat the 6-year-old Illinois boy who was buried for hours in a sand dune said Monday, July 15, 2013, he’s respo
This undated photo provided by the Woessner family via The University of Chicago Hospital, shows Nathan Woessner. The doctor who helped treat the 6-year-old Illinois boy who was buried for hours in a sand dune said Monday, July 15, 2013, he’s responsive and expected to make a full neurological recovery. -- FILE PHOTO: AP
In this July 12, 2013, photo rescue workers with heavy equipment working to free 6-year-old Nathan Woessner who fell into a hole in a massive sand dune at the Indiana Dunes National Lakeshore in Michigan City, Indianapolis. Woessner remained hospital
In this July 12, 2013, photo rescue workers with heavy equipment working to free 6-year-old Nathan Woessner who fell into a hole in a massive sand dune at the Indiana Dunes National Lakeshore in Michigan City, Indianapolis. Woessner remained hospitalised in critical condition on Monday after he was buried for three hours in a sand dune at a popular Great Lakes beach area in Indiana. -- PHOTO: AP

INDIANAPOLIS (REUTERS) - A 6-year-old boy remained hospitalised in critical condition on Monday after he was buried for three hours in a sand dune at a popular Great Lakes beach area in Indiana.

Nathan Woessner sank 3.3m into the dune last Friday afternoon while on an outing with his family, US National Parks spokesman Bruce Rowe said.

Woessner is sedated and on a respirator as medical personnel try to remove sand from his lungs and run other tests, said Dr Tracy Koogler, medical director of Pediatric Intensive Care at the University of Chicago Medical Center.

He was improving faster than expected, she said.

It was unclear what caused the accident, Mr Rowe said. The location where it happened, Mount Baldy, has been closed indefinitely.

Rowe called the incident "unprecedented" in the history of the National Park Service. Indiana Dunes National Lakeshore, along Lake Michigan south of Chicago, is maintained by the National Park Service.

Mr Rowe said one possibility is that the remains of a tree at the bottom of the 126-foot (38-meter) dune created a void under the sand. Rescuers saw what may be traces of a rotting tree where Woessner was found, he said.