The second largest in the world, this massive underground void has its own subway.

This neon lit Georgian cave claims to hold the boulder Prometheus was chained to.

A cluster of historic bathhouses built on the famed hot springs that gave the Georgian capital its name.

You'll find just about anything in this funky Tbilisi flea market.

The charmingly disheveled clock tower of Tbilisi's puppet theater.

Thousands of rusting train cars have been slowly falling apart on the edge of Greece's second largest city.

Hanging over one of Japan's "hidden valleys," these vine bridges must be remade every three years.

Every autumn, this hillside is ablaze with red summer cypress.

Once the most densely populated place in the world, this island is now a ghost town.

World's largest cave was only discovered in 1991, and now offers tours to the public.

Statues of Ramona Quimby, Henry Huggins, and Ribsy the dog in the park where their adventures "really happened."

Whitewashed mosque contains the world's largest carpet.

An abandoned desert hamlet swallowed up by sand.

These paved rills bring freshwater into Altstadt and could spell wedding bells for whoever stumbles into the waters.

Jumping-off point for a climb up Mt. Everest is the most dangerous airport in the world.

Last stop before the top of the highest peak in the world.

This natural Icelandic fissure allows divers to swim right between two volatile tectonic plates.

This half-forgotten fortress was constructed by the Ottoman conqueror of Constantinople.

This shuttered power plant is now Turkey's first industrial archaeology museum.

This canyon is comprised of gorgeous waters and magnificent basalt columns.

The tallest outdoor elevator in Europe rockets passengers on a dizzyingly precarious ride to the top of a mountain.

This 170-year-old Swiss guesthouse built into the side of a mountain is best accessed by a cable car.

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

This was the second largest mercury mine in the world and one of the oldest preserved mine complexes in Europe.