10 Mysterious Underground CitiesS

Humans like to build cities that reach for the sky, but sometimes they also build downward. Whether for protection, or just because they can, we've created many cities beneath the Earth's crust - or dug deep into the rocky faces of mountains. Here are ten of the most awesome.

1. Petra
Built in Jordan in the 2nd century by the Nabataeans, this massive monastery is just one of hundreds of dwellings carved into the rocks. Photograph by Martin Gray/National Geographic

10 Mysterious Underground CitiesS

2. Barry Troglodyte Village
People have lived for hundreds of years in this village, carved into the rocks of a hillside overlooking Rhone, France.

10 Mysterious Underground CitiesS

3. Moose Jaw
Beneath the streets of Saskatchewan, Canada town Moose Jaw, there lurks an extensive network of tunnels, full of what were once shops, factories, homes, and speakeasies. Rumor has it that Al Capone used them for smuggling, and that Chinese immigrants lived and worked there in the nineteenth and early twentieth centuries. There is no solid evidence either way, but you can take a tour. Learn more via Tunnels of Moose Jaw.

4. Leavenworth
Beneath the streets of this small Kansas town, residents recently discovered three blocks of abandoned storefronts. No one is sure what they were used for. Were they the homes of escaped slaves, who hid from authorities? A haven for bootleggers in the 1920s? Or was it simply a part of town that was abandoned and turned into landfill for the Leavenworth we know today?

10 Mysterious Underground CitiesS

5. Montreal
Connected to metro stations and shopping districts, Montreal's "underground city" boasts 20 miles of tunnels, making it the largest such installation in the entire world.

10 Mysterious Underground Cities

6. Derinkuyu
This ancient Turkish city could house up to 50 thousand people, reached eleven stories below ground, and was built in the 7th or 8th century by Christians hiding from persecution. Turkey is full of underground cities, in part because the volcanic rock in the region is easy to carve out.

10 Mysterious Underground CitiesS

7. Cheyenne Mountain
Once home to NORAD, this underground city was built beneath Colorado's Cheyenne Mountain to hold thousands of people and withstand a nuclear attack. Though it was secure during the Cold War, advances in missile technology mean that it's no longer a sure bet when the nukes fall. So NORAD has been moved to a new location, and the city remains standing - some people still live there but it may become a museum soon.

10 Mysterious Underground Cities

8. Beijing Underground City
Like NORAD, Beijing's famous Underground City was built during the Cold War to withstand a nuclear attack. Now the vast network of tunnels has become a tourist attraction. Image via SeraSphere

10 Mysterious Underground CitiesS

9. Portland
In Portland, Oregon on the west coast of the United States, there's a vast network of underground tunnels, former homes and businesses. The place was used almost entirely for bars before and during Prohibition, and earned the nickname "Shanghai Tunnels" because unwary sailors were often "shanghaied" - drugged while drinking, only to find themselves aboard a ship the next day, where they had to work until they hit another port.

10 Mysterious Underground CitiesS

10. Coming Soon: Amsterdam
Here's a future underground city: Starting in 2018, Amsterdam will be building an extensive network of underground tunnels for shopping and parking, right beneath the canals.