LEADVILLE, Colo. (CBS4) – The story behind a sinkhole near Leadville runs deep. A 100-year-old railroad tunnel under the road collapsed.

The sinkhole formed on the highway on Monday and shut down Highway 24 over Tennessee Pass, which connects Leadville and Minturn.

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The tunnel was built in the 1890s to carry passenger railcars from Canon City to Salida and Minturn.

“The original one was built of wood and timber so it would have rotted away over the years,” said Colorado Railroad Museum spokeswoman Sharon McGee.

The tunnel crosses Highway 24 in three places.

The sinkhole is about 100 feet deep and curves under the highway.

“There are chunks falling off. You can hear them,” said Colorado Department of Transportation Engineer Martha Miller. “So that’s why we’ve closed the road, we don’t know how quickly it’s going to happen and how quickly it’s going to occur.”

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(credit: CBS)

Drilling crews are investigating what is around the sinkhole.

“What we’re doing now is drilling a series of holes in an investigation to see if we feel like there are any more in this vicinity. If we don’t find any, we’ll probably reopen the road if we feel like it’s safe,” said Miller.

Engineering efforts are under way to see how the road and the ground below can be stabilized to make sure it’s safe for vehicles.

“We’re going to most likely have some sort of engineering project, an emergency progect, to come in here and fix the void under the road. We’re not going to leave it to the unknown,” said Miller.

The most likely scenario is that crews will use concrete to stabilize the earth around the sinkhole.

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They hope to have a fix in place before next month’s USA Pro Cycling Challenge. The route takes 5,000 cyclists along Highway 24 over Tennessee Pass, which is where the sinkhole is.