This futuristic German spa allows visitors to float in a pool of salt water while zoning out to underwater techno.

Housed in an old British jail, this spot serves up Georgian, Russian, and Ukrainian specialities with a side of Turkish history.

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

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

A marvel of Byzantine engineering under the streets of Istanbul.

A 13th-century Swiss castle houses a collection of artifacts from the Polish diaspora.

A unique collection of Swiss art brut and naïve art displayed in an old warehouse.

The beautiful baroque library of the Hapsburg empire.

A collection of anatomical samples from one of the first anthropologists to suggest that humans reached North America by crossing the Bering Strait.

This tipsy office building in downtown Prague was originally called “Fred and Ginger.”

Around 100,000 bodies are buried below the medieval headstones, many of which mark graves with multiple bodies stacked up to 12 deep.

The hidden lair of an alchemist rediscovered in a 2002 flood.

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

Fantastic lake on the heart of a fault line.

The largest Neolithic ruins in the world hide a notably clean prehistoric metropolis.

The best-preserved ancient theater in the world.

Roman ruins, natural methane fires and a lovely beach.

Flames have been burning within this mountain for more than 2,500 years.

These abandoned Cold War-era bunkers are now a designated refuge for endangered bat species.

This French ghost town from World War II has been left as a memorial to German atrocities during World War II.

This picturesque medieval monastery guards a small trove of historic treasures.

Amidst medieval and baroque buildings, five human-sized gherkins stand tall.

This 19th century cyclorama acts like an analogue time machine to a different era.

Terrifically morbid cemetary and site of famous Paracelsus' grave.