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.

Five sultans and their families rest in eternal splendor, teeming with overlooked beauty and drama.

Renaissance hospital featuring a rotating door into which unwanted babies were dropped.

Legend says this mysterious stone head belongs to a woman who was petrified by an alchemist.

This massive subterranean fissure is known as the "underground Grand Canyon."

On Gellért Hill, this statue depicts the tragic tale of lovers divided by the Danube.

These tall, red sandstone pinnacles outside Moab are often forgotten amid their more famous neighbors.

In the middle of a remote desert town is a surprisingly opulent hotel.

The largest covered bazaar in the world.

The eerie remnants of an ancient forest were hidden under the sand for centuries.

Lost subway station nestled beneath Toronto's busy commuter line.

A replica of the first seismoscope ever invented.

The absurd creation is a compulsory destination for selfies in Buffalo.

A commemorative plaque hides a door that leads down to the New York Public Library's underground book bunker.

Hundreds of caves filled with thousands of centuries-old statues and murals.

The ruins of an Estonian prison are drowning in the quarry lake where the convicts were once forced to work.

At high tide this strange footbridge appears to have no purpose whatsoever.

The cell which saved the life of Ludger Sylbaris, "the man who lived through Doomsday."

The remains of a 37,000-year-old prehistoric society were discovered in this mammoth cave, one of the largest in Asia.

A giant gun barrel in Barbados that once hoped to shoot outer space with science.

This lovely little burbling creek is actually a deadly waterway that has dragged down everyone who ever set foot in it.

A small but immensely extravagant palace built by the "Night King."

Hundreds of dwarf beeches create an unusual, haunting forest in the north of France.