SILVERTHORNE, Colo. -- An Illinois man suffering from severe hypothermia was airlifted to Denver Tuesday morning after his pickup plunged down a river bank near Silverthorne.

Lake Dillon firefighters found the frigid, incoherent man wearing nothing but his boxer shorts, crouched in some willows and rocking back and forth. The man was suffering from advanced stages of hypothermia, said fire spokesman Steve Lipsher.

People suffering from advance stages of hypothermia –- a dangerous loss of body heat -- often behave irrationally and may even begin stripping off clothing, Lipsher added.

Rescuers only discovered the unidentified man because residents of a nearby house noticed that their heat had failed around 1 a.m. They didn't realize that their propane tank had been knocked down a 60-foot embankment until about 8:30 a.m. That's when residents spotted the man's red Toyota pickup at the bottom of the steep hillside above the Blue River and called 911, Lipsher said.

Crews from the Summit County Sheriff's Office search and rescue team, and an ambulance crew helped wrap the man in a heated sleeping bag, strap him on a litter and haul him up the embankment.

He was then flown by Flight for Life to St. Anthony’s Central Hospital in Denver. His name and condition have not been released.

The pickup had turned off the road onto a short driveway, careened around the house and slipped between two telephone poles before slamming into the 500-gallon propane tank atop the embankment, Lipsher said.

The residents suspect the crash occurred around midnight, and somehow the freezing man survived a night when temperatures plunged to 3 degrees.

Sheriff's deputies and rescue-team members performed a search of the area after discovering what they thought were a second set of footprints leading from the crash site. No one else was found.