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

A museum befitting the city responsible for supplying the majority of the world's LSD.

Its buzzing collection honors vibrator history.

Carnivorous plants, medieval aphrodisiacs, and the largest original wood structure glass conservatory in the western hemisphere.

Relics of the 1894 Midwinter Exposition continue to delight visitors today.

The only theatre in the world constructed specifically for sound movement.

A tiny theater hidden in an alley plays a constant stream of short silent films for anyone willing to peer through the peephole.

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

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.

A psychedelic labyrinth on the San Francisco bay.

A huge musical instrument played by the ocean.

Have tea in a cemetery with a dazzling view of Istanbul.

A surprising and unconventional mosque hidden in the subsoil of Istanbul.

Climb a steep hill in 90 seconds on the second-oldest urban subway in the world.

A sixth-century Byzantine cistern hidden below a modern carpet store.

The ornate miniature mansions were built for Istanbul's feathered residents.

A small etching on the white marble parapet was written in runic script by a Viking mercenary.

Walk through the actual strap iron cells and solitary chambers of Arizona Territory’s first prison.

Stock up on all your horror paraphernalia at this year-round Halloween emporium.

A planet-hunting astrograph built in the 1920s to seek out the so-called "Planet X" beyond Uranus.

One of the best-preserved ghost towns in Arizona speaks to the difficulties of establishing mining towns in the 19th century American West.

An out of this world experience.

The sprawling attraction boasts a collection of more than 150 historic wagons.