A self-built castle made by a mysterious man.

A century-old miner's quarters turned mansion.

This famous Nevada gold mine, once a hotbed of Wild West shenanigans, is now open for tours.

This naturally heated mountainous creek has created a group of neon blue pools of potentially deadly boiling water.

Home to the "Circus" trees created by Axel Erlandson.

Once America’s most beautiful army base, this abandoned military fort on the coast of Monterey Bay has been left to the elements.

A popular tourist cave with a dark past.

These millipedes glow not out of attraction, but as a not-too-subtle warning.

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

A Cold War site complete with rusty structures, hiking trails, and beautiful views.

The site of a classic series of Laurel and Hardy pratfalls.

What began with a single, 357-pound chunk of the Canyon Diablo meteorite grew to be an impressive collection. 

This rare example of a "tidefall" is a spectacular drop from the cliffs down onto the beach.

Abandoned bridge hidden deep in the San Gabriel Mountains.

An unfortunate geological resemblance to Satan has labeled this Pasadena gorge as a passage to the underworld.

Finale of "William Tell Overture" can be heard as drivers pass over grooves in the road.

Big gun installations, built over a massive cemetery, now abandoned & hidden in trees.

The now-abandoned tunnels were built for the transcontinental railroad on the route where the first wagon train entered California.

A nondescript building that's been functioning as a speakeasy for nearly a century and a half.

Eons old volcanic activity created one of the most stunning rock formations in the United States.

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

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

Carved by a waterfall in a hard-to-reach rock formation, this heart-shaped rock has inspired romantics for decades.

The seawater playground of gilded-era San Francisco burned to the ground in 1966.