An overlooked and unassuming engineering marvel.

San Franciscans leave gifts and messages for the magical beings that live behind the tiny, mysterious door installed on a tree in Golden Gate Park.

This concrete tangle of square pipes has stood as a proud thorn in the city's side since the '70s.

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

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

A pair of slides and a tree-lined stair corridor have been an urban oasis for nearly 40 years.

The Internet Age began with a transmission from this biker bar.

Three plaques mark the spot where the "forgotten father of broadcasting" worked.

From the sky, a kaleidoscope of color brightens the grey landscape of the San Francisco Bay.

The world’s first stop-motion film was created on the racetrack outside this building.

A sudden forest of traditional Papua New Guinean art works is hidden in a corner of Stanford University.

Internationally-recognized arts center housed in a century-old former military barracks.

A one-of-a-kind collection of a unique medium.

This Southern Californian Conservation Center is home to singing and howling apes.

Steinbeck's old stomping grounds, immortalized in Cannery Row.

One of the most elaborate displays of Jellyfish is on display at the aquarium.

A statue commemorate Tommie Smith and John Carlos's brave protest at the 1968 Olympics, a watershed moment for civil rights.

A pair of pure white felines guard the entrance to a spot of California real estate established as an artist's haven.

An ominous concrete cube cuts a dystopian silhouette atop this California mountain.

This old-world mansion with lush gardens is a gem in the heart of Silicon Valley.

Rodin's famous depiction of Dante's 'Inferno', amidst a sculpture garden of many of his other works.

This odd California folly seems like it was almost designed to accumulate urban legends.

A trail weaves through the verses of a poem illustrated by stone sculptures inspired by Native American pictographs.

This small monument to the great inventor contains a time capsule and a Wi-Fi hotspot.