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

An underground mansion with fruit-bearing trees and grapes growing beneath the surface of central California.

World's first "penitentiary," meant to be humane, drove men insane.

America's most famous museum of medical oddities is home to the remains of Albert Einstein's brain.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

Home to items never before seen by the public.

Mutant vegetables, transgenic mosquitos, and atomic rodents span the walls of this one-of-a-kind museum.

A man-made island in scenic San Francisco Bay.

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

A rough granite monument marks the spot where Europeans first caught sight of the San Francisco Bay.

The old Niles Canyon Aqueduct in the San Francisco Bay Area is a bemusing and secret trail through nature for wayward travelers.

Apple memorabilia in an undisclosed warehouse, hidden for undisclosed reasons.

Home to the "Circus" trees created by Axel Erlandson.

A tribute to the life and work of the beloved "Peanuts" cartoonist.

A strange house built only at night, with rumored mystic inspiration.

The haunting ruins of Jack London's dream house.

This abandoned boat was left in place thanks to its rotting beauty.

A "gravitational anomaly" located in the redwood forests just outside of Santa Cruz, California.

A vast city of the dead, where the deceased outnumber the living by more than a million.

This odd little museum holds exhibits on everything from blacksmith equipment to hair jewelry.

A huge musical instrument played by the ocean.

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

This partially buried wreck is one of the most complete of its kind but is only revealed when the sea allows.

An art collective teaching the more fiery, metal-y art forms.