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

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

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

The randomness of this wall of lava lamps helps encrypt up to 10 percent of the internet.

San Francisco's newly reopened hands-on science museum.

Tiki luxury on Nob Hill.

A walk-in working camera obscura produces 360-degree live images of the San Francisco coast.

Its buzzing collection honors vibrator history.

Carnivorous plants, medieval aphrodisiacs, and the largest original wood structure glass conservatory in the western hemisphere.

Relics of the 1894 Midwinter Exposition continue to delight visitors today.

The only theatre in the world constructed specifically for sound movement.

A tiny theater hidden in an alley plays a constant stream of short silent films for anyone willing to peer through the peephole.

A 140-year-old castle with underground caves hidden in San Francisco.

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

Graffiti-covered ruins that were once part of a farm owned by a mental hospital.

360-degree virtual reality chamber physically immerses scientists into their data.

The oldest pictographs on this decorated rock formation in California's Carrizo Plain date back as many as 4,000 years.

The first attack on the continental U.S. during WWII occurred off the coast of a quiet California golf course.

Happy harbor seals frolic and snooze on this warm, sandy stretch of coast.

The home of one of California's most important horticulturalists wilts amidst a lovely park.

A popular winter vacation destination for migrating monarch butterflies.

Colorful Native American paintings are hidden within this small cavern.

Acres of natural wonders left behind by a socialite-turned-gardener.

The beautiful behemoth is one of the largest trees of its kind in the United States.