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

The cave of a 12th-century hermit is still visited and venerated today.

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

After years of being casually peed on by humans, this tree is peeing back.

This demonic monument commemorates a carving destroyed in World War II.

This German bone house contains the remains of some 20,000 souls.

This memorial to a family's dead son has survived a century of hurricanes and earthquakes.

This colorful gate stands as an entrance to the walled city of Old San Juan.

La Cara del Indio honors the Taíno chief who fought bravely against the Spanish invasion of Puerto Rico.

This bleach-white graveyard overlooking the ocean is accessed through a tunnel.

Legend has it this historic port was a popular hideout and smuggling spot for pirates.

This ''hidden'' 19th-century bridge played a simple, yet important role during the Spanish-American War.

Invading U.S. troops were first seen from this now-abandoned tower, leading to the Spanish-American War.

The ruins of the first Spanish settlement in Puerto Rico.

There's an exclusive Beefeaters pub hidden in the Tower of London.

The 19th-century pharmacy (now a used bookstore) was featured in James Joyce’s "Ulysses," and has changed very little to this day.

Mirrored floors and arched shelves create the illusion of an infinite tunnel of books.

An atmospheric, medieval-themed book store hides in a 13th-century underground space.

This historic, palatial theater is now one of the world’s most beautiful bookstores.

This once-derelict building is now one of the most beautiful bookstores in the world.

Thousands of well-dressed mummies below a monastery.

Visit the ancient, mysterious Roman catacombs ... in the Netherlands.

A burial vault located under Tiber Island in the middle of the Italian capital.

These lesser-known Maltese catacombs date back over 1,700 years.