A psychedelic labyrinth on the San Francisco bay.

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

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

A huge musical instrument played by the ocean.

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

This eye-popping Icelandic crater lake is surrounded by red volcanic rock.

Two stunning 17th-century libraries and a cabinet of curiosities.

Skeletons dressed in extravagant 18th century royal costumes cavort in the aisles of this German basilica.

A lovers' beach, tucked below the surface of the island, provides a safe haven for romance.

Monasteries on rock pillars, once accessible only by frayed ropes.

A beautiful but toxic fountain of mercury.

Fall down the rabbit hole into a surreal journey of human consciousness "from ass to soul."

A collection of 100 brains preserved in jars on the UT campus.

This abandoned coliseum was intended to be a grand Nazi rallying ground before it was halted at the end of the war.

Ruins of a German Castle on top of a towering stone pillar.

Silent memorial, using bone conduction to take people back to the Dresden Bombing of 1945.

When the rain falls, this drain system turns into a musical instrument.

Slaughterhouse Five opens its dank doors to fans of Kurt Vonnegut.

The largest treasure hoard in Europe is contained in a castle that was bombed out during World War II.

Terrifically morbid cemetary and site of famous Paracelsus' grave.

Ugly 18th-century dwarf statues living in a perfectly landscaped Austrian garden.

This medieval castle overlooking the Elzbach river looks just like it did hundreds of years ago.

A preserved college lock-up that recalls a clubhouse approach to discipline.

Thousands of tourists and residents in Cologne affix "love padlocks" on the Hohenzollernbrücke Bridge to symbolize their affections.