Giant Sinkhole Determined To Be Century-Old Train Tunnel; CDOT Closes Colorado's US-24 Highway 'Indefinitely' (PHOTOS, VIDEO)

A giant sinkhole that opened up alongside US-24 highway near Leadville on Monday has forced the Colorado Department of Transportation (CDOT) to completely shut the highway down to all traffic north of Leadville at mile marker 165 "indefinitely."

After several engineers, maintenance supervisors and geological experts examined the sinkhole on Monday, it was determined that the sinkhole is actually a century-old railroad tunnel that collapsed decades ago, CDOT said in a press release.

The depth of the hole is 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.

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7News spoke with Rex Goodrich, a CDOT engineer who explained why engineers decided it was safer to shut the highway down completely. "It's the unknown. Twenty feet away, there could be another hole like this ready to break through," Goodrich said to 7News.

The collapsed tunnel was reportedly once part of the old Denver and Rio Grande Western's Tennessee Pass Route. It was constructed as part of the Royal Gorge Railroad route in the 1880s, The Denver Post reports.

According to 9News, Union Pacific now owns it and said the tunnel has been abandoned since the 1950s.

All of US-24 is not closed between Red Cliff and Leadville, just the section near the sinkhole at mile marker 165. CDOT is recommending using State Highway 91 as an alternative route, as there is no estimated time of reopening.

The Colorado Department of Transportation (CDOT) posted photos of the giant sinkhole on their Facebook page.

LOOK: Giant 45-foot-deep sinkhole alongside US-24 near Leadville, via CDOT:

Additional photos of the US-24 sinkhole and more giant sinkholes from around the world: