CENTRAL CITY, Colo. (CBS4) – A massive crater alongside the road in between the cities of Black Hawk and Central City has traffic moving along a bit slower than usual after crews had to close a lane.

The sinkhole happened at the end of Gregory Street just east of Central City. The asphalt on the side of the road gave way to a small cavern early Saturday morning. By midday the cavern had turned into a crater.

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Officials said an old mine shaft caved in. There’s some speculation that recent, heavy rains and heavy Memorial Day weekend traffic may have been a contributing factor.

The shaft is about 30 feet wide by 50 feet deep. Crews will fill it in with large rocks and then add filler to the rock for stabilization.

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Experts say such collapses aren’t uncommon in Gilpin County.

“It’s known for being one of the richest square miles on Earth, maybe in the United States. I mean this place was hopping back in the early 1900s,” Deb Zack with the Division of Reclamation, Mining, & Safety said.

The sinkhole near Central City (credit: CBS)

Crews don’t believe that the shaft continues under the road but they’ll be keeping a close eye on the spot.

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Traffic will be down to one lane in the small section in between the two towns until crews can get the hole filled in and stabilized.