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

Part mural and part sculpture, this work of art sits in the middle of the Promenade, as if swimming in a river of asphalt towards the ocean.

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.

A hidden subterranean installation offers a view of the sky.

Art in the form of a winding, man made crack in stone.

A huge musical instrument played by the ocean.

Get lost in this haunting fairytale art walk through a man-made eucalyptus grove.

An overlooked and unassuming engineering marvel.

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

With the touch of a button, you can read a randomly selected short story.

Adolph Sutro's little-known, geographically inaccurate, deprecated monument.

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

The giant pair of sexy legs sticking out the window of this costume store have become its main draw.

San Francisco's oldest surviving monument and meeting place for 1906 survivors.

Gold Rush-era tombstones line the park's paths.

The miraculous hydrant that saved the Mission District in 1906.