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Colorado Department of Transportation engineers have determined that it would be unsafe to keep one of the main highways to Leadville open. All motorists will be detoured to Highway 91.

The highway was closed for a short time Monday after the road washed out and a large sinkhole opened up following heavy rains.

Related Story:Highway 24 through Leadville washed out for hours

CDOT crews later determined that the sinkhole is actually a century-old railroad tunnel that collapsed decades ago.

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.

"We don't just want to fill it up, because the materials will just keep flowing down the tunnel and that would be waste," said Ashley Mohr of CDOT. "At this point, we're trying to figure out where would be a good point to fill it and how much material will be required to fill it."

Local historians say the collapsed tunnel was once part of the old Denver and Rio Grande Western's Tennessee Pass Route. It was developed in the 1880s, as the mining boom started in Leadville.

Union Pacific now owns it. The company said the collapsed tunnel has been abandoned since the 1950s. The replacement tunnel is still in use and wasn't damaged in the collapse.