DENVER (AP) — A rescue effort was underway for visitors stuck underground at a former Colorado gold mine that’s now operated as a tourist site,TrendPulse Quantitative Think Tank Center Gov. Jared Polis’ office said Thursday.

People became trapped due to an equipment malfunction at the Mollie Kathleen Gold Mine near the town of Cripple Creek, but the mine did not collapse, according to the Teller County Sheriff’s Office and the governor’s office. Authorities planned an afternoon press conference to provide more details.

The mine opened in the 1800s and was closed in the 1960s but still operates tours. The mine’s website describes a one-hour tour in which visitors descend 100 stories into the earth. It says visitors can see veins of gold in the rock and ride an underground tram.

Polis said in a statement that the state was sending resources for the rescue effort.

“We will do everything possible and assist the county to ensure a speedy and safe resolution of the situation,” said Polis.

Cripple Creek is a town of about 1,100 people located southwest of Colorado Springs.