Alleyway plaque commemorating the death of a fictional character in The Maltese Falcon, not far from the author's home.

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

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

Bring your own cardboard to the slippery slopes of concrete hidden in a neighborhood park.

An artsy hidden staircase leads to breathtaking views of San Francisco.

The rest of him lived another 61 years and was buried elsewhere.

Law-protected wreckage serves as a peculiar memorial for an almost forgotten World War II tragedy.

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

A huge musical instrument played by the ocean.