Moonbows over Kentucky.

This large midwestern cavern features impressive rock formations and an underground waterfall.

These Great Lakes sea caves accessible by boat in the summer turn into temples of ice reachable by foot in the winter.

Friction between grains of sand cause a curious "singing" sound.

A secretive collection featuring objects that were designed specifically for U.S. intelligence operations.

Hidden in plain sight on the Hudson River is an abandoned train station where nature has taken over.

The artist James Turrell is repurposing an extinct volcano crater as an observatory to experience the cosmos.

The remnants of two World's Fairs are here, complete with a 12-story globe, a mini-Manhattan, and a UFO-shaped pavilion.

America's only nuclear missile silo open to the public.