At the crypt of St. Mary of Eulogies and the Dead you are left alone to ponder mortality among piles of skulls.

This hidden alley near the location of Rome's first theater was the site of a curious miracle.

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

The Pope’s secret escape route.

An unremarkable wooden wheel built into the side of one of Rome's oldest hospitals was part of a policy to reduce infant exposure.

One of the most important yet well-hidden museums dedicated to the history of medicine.

Rome's semi-secret peephole vista is also a former Crusader stronghold.

A subterranean relic of the Spanish Civil War hides below this picturesque park.

This top secret Chinese military megaproject is the world’s largest human-made tunnel structure.

A tour of underground Berlin takes you through WWII air raid shelters and tunnels used to smuggle people out of East Germany.

A slice of central Asia in the middle of Berlin.

One of Antoni Gaudí's least-visited works, an homage to Catalan patrimony that mixes neo-Gothic with Modernista forms.

One of Europe's largest supercomputers is housed inside of a disused chapel.

This ancient ruined monastery has been destroyed by fire so many times that it seems like God hates it.

A colossal sculptural lion built by the same man who designed the Statue of Liberty.

The remains of this castle, known as the Witch’s Eye, watch over the Thur Valley.

Before the electric telegraph, a chain of stone towers carried messages across France.

A sandstone block marks the alleged spot where the legendary writer was conceived.

An ancient monastery, a locked room, and a mysterious book heist.

This beach provides a glimpse of the ancient seafloor without getting wet.

Anyone wanting to know what a dwarven mine sounds like need only visit the tunnels of this New Zealand artillery compound.

These phallic stone spires once served as Tolkien's Paths of the Dead.

Some of the clearest water anywhere in the world.

Revitalization efforts converted a former U.S. military heliport into a sprawling urban park.