In a 345-million-year-old cave that's 200 feet underground, dinner awaits.

These otherworldly caverns form Earth's oldest known cave system.

A massive cave complex boasts some of the clearest water (and most charming bats) in the world.

Every drop of D.C. tap water flows through this old waterworks.

A fountain standing as a testament to a doomed alliance.

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

Ride a boat through this magnificent underground world, which now doubles as a theater.

These paved rills bring freshwater into Altstadt and could spell wedding bells for whoever stumbles into the waters.

It's not quite as old as its Roman appearance suggests, but this piece of infrastructure served Guatemala City for centuries.

This Arab fortress was once the largest citadel in Western Europe.

A well preserved medieval fortress in Vojvodina.

This magnificent castle grew from a small Moorish fortress, and reportedly inspired two iconic Disney castles.

This enormous secondhand bookstore inside a Victorian train station sparked the "Keep Calm and Carry On" craze.

The sign at the garden gate reads: "These Plants Can Kill."

This giant wooden treehouse within a sprawling garden has its own eatery.

This restaurant was removed from the famed sister ship of the Titanic and placed within a hotel.

This iconic 1,000-year-old medieval castle is a favorite of the film industry, most notably as the location for Hogwarts in Harry Potter.

A remote and slightly overgrown park filled with replicas of world landmarks.

The field of tiny caves that mark this Chinese cliffside are each their own historic Buddhist temple.

The home where a lovestruck Fitzgerald wrote his first published novel, "This Side of Paradise."

Gladiators battle and chariots race daily in the ruins of a once-great Roman city.

Venice's "Island of the Mad."

As can be expected from a centuries old church built in a canal city, the undercroft of this house of worship is beautifully flooded.

A small museum filled with over 1,000 clocks, and one particular clock calibrated to run until the year 9999.