DENVER (KDVR) — Authorities in Colorado say multiple teenagers have been rescued from an abandoned missile silo. One of those teenagers, an 18-year-old, has been listed in critical condition after falling roughly 40 feet.

The Arapahoe County Sheriff’s Office and several other agencies were called to the abandoned missile silo near Deer Trail, a small town located about 50 miles east of Denver, around 3:30 a.m. local time Sunday morning to rescue the teens.

The sheriff’s office told Nexstar’s KDVR that eight teenagers had trespassed onto the land. The Sable Altura Fire Rescue, a team assisting on the call, later said that four teenagers were rescued from the missile silo.

Recounting the rescue during a press conference Sunday, officials said they spent almost two hours trying to find the teenagers. Communication was difficult within the silo due to a lack of cell reception.

The operation was described as one of the most dangerous rescues authorities have ever done.

“We searched for a while,” Rich Solomon, fire chief for Sable Altura Fire Rescue, said during Sunday’s press conference. “It’s a maze of tunnels, twisted metal, an old railway that the military must have used,” he said while describing the silo, which has likely been abandoned since the 1960s.

Solomon said they were able to find one teenager who had been looking for them. They then found the critically injured teen who had fallen roughly 40 feet down a shaft and a friend who stayed with him.

Just before 9 a.m. local time, authorities were able to rescue the critically injured teenager using “technical handling.” He was then hand-carried to a waiting helicopter, which air-lifted him to a local hospital.

Authorities told KDVR that the teen is expected to survive but it’s unclear what injuries he sustained. Another rescuer said the teen is “lucky to be alive.”

In total, Sable Altura Fire Rescue said it took nearly five hours for the teenagers to be rescued from the abandoned missile silo.