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 monumental cross hides in the trees in Golden Gate Park.

A tribute to the artist and activist who created the rainbow pride flag.

The only diamond mine in the world where you can keep what you find.

This historic hotel on Bathhouse Row in Hot Springs National Park has a storied history of gangsters and baseball greats.

These subterranean storage areas hold emergency water reserves.

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

An "anarchical" no man's land, complete with garbage sculptures and a hobo-run "Landfillian Library."

A favorite haunt of dinosaur loving children and hallucinogen loving hippies alike.

The oldest lawn bowling club in the United States.

This turn of the century masterpiece has a mysterious goaty interloper.

An overlooked and unassuming engineering marvel.

A masterpiece in Golden Gate Park by virtue of the artist’s debt.

San Francisco's first World's Fair, dedicated to the wonders of California.

An elaborate gift from Claus Spreckels is now a piece of San Francisco history.

Barely visible foundations remain where Stow Lake’s “observatory” once welcomed visitors.

This print shop boasts one of the largest pinball arcades in San Francisco.

This 19th-century church is now a popular roller rink.

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

This painted marvel is San Francisco's 400-foot ode to urban biking.

This one-block alleyway is stuffed with street art that is colorful, cultural, and intensely political.

The miraculous hydrant that saved the Mission District in 1906.

Watch the newest land on the planet form before your eyes.

This black shoreline is made up of exploded lava particles.