The world's longest known cave system.

In the middle of an extremely unforgiving and desolate high desert is the most secret military facility in the world.

Stepping away from the glitz of mobsters and casinos to learn about the other history of Vegas.

The ancient dwellings of the Puebloans, located in the cliffs of majestic mesas.

The rocks in this arid basin leave trails of movement behind them.

The only structure left with direct ties to the Salem witchcraft trials of 1692.

A bizarre house filled with an astounding array of collections.

A crumbling hospital from the 1850s on Roosevelt Island.

Clap your hands at the base of the pyramid, and the song of a sacred Mayan bird will echo through the air.

Every equinox this Maya pyramid puts on a spooky ancient light show.

No telescopes, no power, no gift shop: The Observatory of Chichen Itza, one of the world's oldest observatories, takes astronomy back to basics.

The dance of this huge clock is a magnificent and eerie commentary on good and evil in the history of humankind.

Edinburgh's oldest statue hides within a parking lot behind the city's iconic cathedral.

This 16th-century close preserves an unusual piece of Edinburgh's transportation history.

The 20th-century chapel’s intricate woodwork includes amazing details, most notably angels playing bagpipes.

A verdant 17th-century-style garden hidden along Edinburgh’s Royal Mile.

A series of three shiny letters mark the boundary of an old refuge for those being pursued by debt collectors.

Overlooking the burial ground is a tower designed to prevent thieves from snatching the recently deceased.

The location of this historic castle was inspired by a donkey.

This charming old building's loose connection with the Scottish Reformer is what saved it from demolition after it split apart.

Every St. Andrews student knows to avoid this inscription for fear of a martyr's curse.

The world's shortest street was created by an awkwardly placed hotel.

This tin chapel was built by POW's during World War II and is painted to look like something much more grand.

Touching this 18th-century Scottish philosopher’s toe allegedly conjures good fortune.