This whimsical castle of grottoes was the labor of love of a French postman, and one of the greatest achievements of outsider architecture.

An encyclopedic collection of mammal penises.

This ancient Greek place of assembly and marketplace is being revealed layer by layer below the modern Athens cityscape.

This legendary city has been a tourist attraction for centuries.

The hallucinogenic center of the ancient world.

This highest bridge in the world spans such a great chasm that it had to be established using rockets.

For those with no fear of heights: a skywalk 4,700 feet up on a Chinese mountain.

A tunnel dug through the side of a mountain by hand provides convenient yet death-defying access to a small village in China.

These former Cold War launch sites have been preserved for tourists to see where the button might have been pushed.

Guests become prisoners on a visit to Latvian prison.

This abandoned secret city is a crumbling wonderland of Soviet ruins.

In the land of the Light Brigade stands a testament to the Cold War.

A bizarre collection of gifts to North Korean dictator Kim Jong Il from the despots of the world.

This abandoned-looking amusement park is frighteningly open for business.

North Korea's massive "hotel of doom."

An impactful museum ensures notorious war crimes disguised as "medical experimentation" won't soon be forgotten.

One of the world's largest ice festivals dazzles visitors for over a month with radiant sculptures and carvings.

The sound of swaying stalks in this stunning grove has been named a governmentally recognized sound.

Below a historic English market town lies a secret underground city complete with kitchens, laundries, storerooms, and an underground lake.

This decrepit abandoned building is made all the more eerie by its dark history.

Decades after testing the polio vaccine on unwitting patients, this historic mental hospital sits in ruin.

A long abandoned hospital building on the outskirts of NYC.

The walls and ceiling of this Polish church are decorated with thousands of skulls, with another 21,000 skeletons just below.

Bizarrely shaped rock formations in northwest Bulgaria come with equally bizarre legends.