EUREKA, Utah (AP) — Underneath the mountains and deserts of the U.S. West lie hundreds of thousands of abandoned mines, an underground world that can hold serious danger and unexpected wonder.

They are a legacy of the region's prospecting past, when almost anyone could dig a mine and then walk away, with little cleanup required, when it stopped producing.

In Utah alone, the state is trying to seal more than 10,000 open mines with cinderblocks and metal grates after people have died in rock falls and all-terrain-vehicle crashes and from poisonous air over the past three decades. Just this month...