At this time, CDOT engineers have determined that it would be unsafe to keep the highway open, so all motorists will be detoured to State Highway 91.

"It’s the unknown," said Rex Goodrich, a CDOT engineer. "Twenty feet away, there could be another hole like this ready to break through."

Crews had marked and barricaded the sinkhole from the highway, but after continued evaluation of the area, were concerned that the sinkhole could expand and did not want to risk lives to keep the highway open.

Looking down the sinkhole into the railroad tunnel. At this time, there is no estimated date of reopening, and the detour will be in place around the clock.

After thorough examinations of the sinkhole Monday involving a variety of engineers, maintenance supervisors, and geological experts, it was determined that the sinkhole is actually a century-old railroad tunnel that collapsed.

The depth of the hole is currently estimated to be about 100 feet, and since the depths reach so far into the earth, much of the soil was still frozen until very recently. When the soil thawed, the hole was exposed.

The tunnel was built sometime between 1900 and 1910, according to CDOT spokeswoman Ashley Mohr.

Mohr explained that the tunnel's wooden supports collapsed decades ago. CDOT thought the tunnel had settled and built over it many years ago.

However, with the warmer weather this year, the frozen soil deep in the tunnel thawed and the hole was exposed.

CDOT said the hole is currently estimated to be about 100 feet. Officials said the tunnel is so deep, engineers still found some ice inside.

"We've had problems in that area in the past and this may be the cause of it," said Mohr.

The collapsed tunnel was the first tunnel built under Tennessee Pass. Union Pacific replaced the tunnel with another one about 600 yards away. It is re-enforced with concrete.