The second-largest cave chamber in the world was discovered in 1898 by a 16-year-old and a friend known as "Pothead."

Final resting place of Geronimo, Quanah Parker, and the first atomic gun ever fired.

Abandoned silver mine strewn with wrecked and twisted structures.

Nature's biggest mammal swims in landlocked Oklahoma.

An oil baron's wild paradise and the winner of an ill-fated air race.

An acoustic anomaly with a mysterious cause.

San Francisco's oldest surviving monument and meeting place for 1906 survivors.