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

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

Most-visited and most-photographed slot canyon in the American Southwest.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

Each year melting snow turns these Austrian hiking trails into scuba zones.

The hottest place on the planet.

One of the most remote and isolated villages on the Arabian Peninsula.

A city built from mud and known as the "Manhattan of the desert."

These spooky and unusual stone formations create an underworldly landscape.

An eccentric company offers eccentric millionaires the opportunity to have spaceflight-related experiences.

This cavernous swimming spot doubles as a home to the largest known colony of bats in Central Asia.

Breathtakingly blue walls shimmer inside the "Glacier Behind the Town."

One of the first hotel rooms in a pod.

Intensely colorful calcite pools in Southern China.

"Excuse me sir, your submarine is blocking my building."

The Yonaguni Monument, a natural formation or the Japanese Atlantis?

The abandoned hospital whose elaborate ghost stories cover up the dirty truth - an uneventful sanitation issue and other mundane reasons for its demise.

An collection of Chinese propaganda posters from 1949 to 1979.

Monastery, army base, and infamous Soviet Gulag in the far Russian north.

A former imperial sea-fortress and Soviet prison.

Museum celebrating the achievements of the Soviet space program.

Elaborate series of tunnels where traces of mass suicide are still visible.

This WWII-era military installation once held an entire city within its walls - now, its walls are falling down.

This ghostly Alaska village still clings to a steep cliffside despite having been abandoned for over 50 years.