A steampunk wonderland that preserves America's history of industrial innovation.

At this ancient settlement at the foot of Mount Babadağ, honeycomb tombs are carved into a round cliff.

This hidden gem is bathed in beautiful natural light.

This ancient subterranean megalopolis is known for the hot oil traps that were installed above each of its entrances.

Mausoleum and Holy Seat of Antiochus I of Commagene.

Standing proud atop a Turkish bluff, this Ottoman palace has retained its grandeur despite being abandoned for centuries.

While not Turkey's deepest underground city, this subterranean redoubt is the largest.

The deepest underground city in Turkey was designed to protect 20,000 people behind massive stone doors.

Naturally protected city carved from a rock face.

Live a night as a Byzantine King, with a minibar and private jacuzzi.

The ornate miniature mansions were built for Istanbul's feathered residents.

A Lycian city of the dead carved into the hillside of Southern Turkey, complete with startlingly human touches.

These tombs were built into the side of a mountain so that ancient angels could reach the dead.

A marvel of Byzantine engineering under the streets of Istanbul.

In this historic region, homes and churches are carved into ancient volcanic rock formations.

The bizarre visions of H.R. Giger, tucked away in a Swiss medieval city.

A familiar otherworldly creature guards this 12th-century Scottish abbey.

This futuristic German spa allows visitors to float in a pool of salt water while zoning out to underwater techno.

Giant Redwood-themed cafeteria operates under the golden rule, "Dine Free Unless Delighted."

During prohibition, corrupt city officials ran drinking dens under the streets of Downtown Los Angeles.

World's largest collection of serial killer artwork and other macabre exhibits.

The restored and active Victorian-era final resting place of San Francisco.

Carnivorous plants, medieval aphrodisiacs, and the largest original wood structure glass conservatory in the western hemisphere.

A more than 260-year-old crypt built on Boston's oldest English burial ground.