1. Derinkuyu, Turkey
This ancient multi-level underground city could accommodate up to 20,000 people.
2. Beijing Underground, China
Built for the purpose of warfare, now serves as a tourist attraction.
3. Coober Pedy, Australia
Known as the 'opal capital of the world', the majority of the residents live underground to escape the extreme heat.
4. Wieliczka Salt Mine, Poland
This underground city consists of a labyrinth of tunnels, the oldest of which date back to the 13th century.
5. Seattle Underground, USA
After the Great Fire of 1889, Seattle was rebuilt over the top of the ruins, creating an underground city.
6. Edinburgh Vaults, Scotland
Once used as residences for the poor and even as venues for illegal activities, they are now a popular tourist spot.
7. Burlington Bunker, England
This city was designed to house the UK government in case of a nuclear war.
8. Petra, Jordan
This archaeological city is famous for its rock-cut architecture and water conduit system.
9. Catacombs of Paris, France
This underground ossuary holds the remains of more than six million people.
10. Shibaozhai, China
A 12-story pavilion was built against a 200 meter cliff in the Qing Dynasty.