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

San Francisco's urban bison herd has persisted in the face of extinction, development, and disease.

The secret city built by the Manhattan Project.

This massive redwood tree has a hole in the base big enough to drive through.

A phenomenal assembly of pueblos in New Mexico is the most complete example of ancient ruins north of the border.

The longest living non-clonal organisms on Earth.

The second largest waterfall by volume in the U.S., Willamette Falls is outdone only by Niagara in the amount of water that is pushed northward.

A French chateau, failed utopian community, odd museum, and Stonehenge replica in rural Washington State.

Hear the quietest sound from across the dome.

A wall of chewing gum moonlights as collective art.

Dyed by iron oxide over centuries, these sandstone rocks rise thousands of feet into the sky.

The lush coastal city of Astoria is riddled with forgotten secret sidewalks.

Aqueducts have dramatically changed this old lake, now home to tufa towers and its very own species of tiny brine shrimp.

An extraordinary mixture of natural beauty and man-made phenomena.

The last remaining relic of San Francisco's glittering 1915 Panama Pacific Exposition.

Trails through lava-carved tube-caves display sparkling bacteria and a rich history.

This lake in the Cascades lives up to its name, disappearing down a hole every spring.

The eerie remnants of an ancient forest were hidden under the sand for centuries.

Canyon walls shrouded in ferns.

This charmingly homemade amusement park has been growing for 40 years.

The one-time watch house for incoming ships at the Golden Gate now stands abandoned and hidden in trees.

One of the most scenic stretches of road in America winds between some of the country's oldest trees.

Rustic natural hot springs in the deciduous rain forest.

See massive fossils that are over 200 million years old in northeastern Arizona.