In 1983, the E.P.A. listed the area as a Superfund site because of mine tailings and runoff. Three years later, after a sharp decline in the price of molybdenum, the Climax mine shut down, stripping Leadville of its heart.

“So many people had to leave the community,” said Bob Elder, a retired mining engineer. “A lot of us felt lost. There was no life left here for quite a while.”

The town has recast itself as a tourist destination  North America’s highest incorporated city, at 10,152 feet, where people can peruse its tiny Victorian-style main street, drink at the Silver Dollar Saloon or ride a scenic railroad. Miners have been replaced by Mexican immigrants, who commute over the mountains to work at ski resorts.

Abandoned mine shafts honeycomb the surrounding hillsides. The old drainage tunnel, built by the federal government in 1943 to drain hundreds of these shafts, began falling apart in the 1970s, causing water to pool. In 2005, the E.P.A. offered to start pumping the clogged water toward a Bureau of Reclamation plant, which treats the water flowing through the tunnel; but the bureau contended that the additional water was part of the E.P.A.’s Superfund cleanup responsibility. A plan for the state to take over the plant subsequently fell through.

Last year, the warnings grew louder. Brad Littlepage, who manages the Bureau of Reclamation treatment plant, says he told supervisors, to no avail, about the tunnel’s deterioration. The risk, as he and other experts saw it, was that pressure from the backed-up water had become so intense that it threatened to burst through the blockages and cascade out of the tunnel.

Image Leadville, Colo., is a historic former mining town. Credit... The New York Times

A spokesman for the Bureau of Reclamation, Peter Soeth, said that Mr. Littlepage’s concerns had been considered but that the agency’s experts had concluded that the tunnel’s condition did not pose an immediate threat.