Bisbee's underground mine tour may be closed.

Bisbee officials said the mining company that owns the property has notified the city that it is terminating the lease for the popular Queen Mine Tour because of safety concerns.

Last year, 43,000 people took the tour, and Bisbee Mayor Adriana Badal said the copper mine is part of city’s identity.

"It is a significant part of our community," Badal said. "It is what identifies us in Bisbee as a people. It’s representative of the mining history of not just Bisbee, but the mining history of Arizona and the country."

Bisbee is trying to work with Freeport-McMoRan, which owns the mine, to find ways to keep it open, Badal said.

Freeport-McMorRan said the safety issues include the presence of radon gas, which is caused by the breakdown of uranium in soil, rock and water. Radon gas naturally occurs in the environment and the U.S. Environmental Protection Agency warns exposure to high levels of radon can lead to lung cancer.

The lease termination would take effect in 60 days. Former Queen Mine owner Phelps Dodge Corp. ceased mining operations there in 1975.