This 19th century granite fortification is one of the most well preserved forts in New England.

This Roman underground canal was the longest tunnel ever built until the late 19th-century.

The Kirkland Monument remembers a selfless Civil War hero who braved the battlefield to give water to his dying enemies.

The remains of a failed attempt at medieval nerdery are hidden deep in the woods of Virginia.

This ramshackle ghost town is comprised of mining-era buildings collected from around the region.

The last occupied troglodyte dwellings in England weren't abandoned until the 1960s.

This 13th-century monastery chiseled into the side of a mountain is equipped with a bridge that can be lifted to protect against raids.

The enormous 250-room Vanderbilt mansion conceals hidden doors and secret passageways.

Richard III supposedly disposed of his nephews' bodies here in an effort to seal his claim to the throne.

A historic custom house with ties to George Washington is now a shrine to local nautical history.

Herman Melville wrote 'Moby Dick' in this landmark 18th-century house surrounded by farmland.

The Denver airport is guarded by a 32-foot-tall sculpture of a demonic horse.

Named for the biblical warrior, the largest castle in Israel actually dates to the Middle Ages.

Heralded as the world's first theater, these ruins are often overlooked as they reside in the shadows of the Acropolis.

A submerged archeological park still holds the wonders of a Roman Sodom.

These remote Florida islands have a history of sea turtles and sunken treasures, and one of the world's largest coastal brick fortresses.

This 170-year-old Swiss guesthouse built into the side of a mountain is best accessed by a cable car.

A cavernous three-story, 43,800-square-foot basement that was forgotten about for 60 years.

The only diamond mine in the world where you can keep what you find.

A legal loophole makes it possible to get away with murder within this 50-square-mile section of Yellowstone.

Far from the shore, this sandy victim of the Skeleton Coast looks like it was plopped in the middle of the desert.

One of the oldest seagoing boats in the world can be found at the Dover Museum.

She become one of the first women to serve in the United States military by secretly dressing like a man.

A popular 19th-century natural landmark is now hidden within a sprawling forest.