No one knows exactly how deep the world’s deepest geothermal hot spring is.

Remnants of the Nazis' occupation of Denmark linger along the sand.

An extensive network of secret tunnels was created for the Nazi project "Riese."

A mysterious structure left behind on the Polish mountainside, sometimes called "Hitler's Stonehenge".

An underground city beneath Krakow's postwar socialist utopia.

The former Nazi concentration camp stands as a museum to remember this dark chapter in European history.

One of the largest amphitheaters in Europe is a former Nazi gathering place that still stands today.

An abandoned Nazi explosives plant founded by the inventor of dynamite is now an underground museum.

A tribute to the Polish insurgents who fought in the failed attempt to end their city's Nazi occupation.

Dubbed "Wistom" by locals, this abandoned factory is an urban explorer's dream.

The shattered remains of the world's once tallest structure.

A monument in the Belarusian city of Polotsk claims to mark the exact midpoint of the European continent.

A huge Soviet-era robotics factory in the outskirts of Pripyat, now long-abandoned.

The ghost town left by the worst nuclear disaster of all time is being taken over by nature and urban explorers.

One of the most poignant ruins of the Chernobyl nuclear disaster, the local football team's stadium has been quickly reclaimed by nature.

This attraction was abandoned after the Chernobyl disaster.

The dining hall that nourishes workers in Chernobyl's Exclusion Zone is also open to visitors.

A monument to Lenin has been transformed into a statue of everyone's favorite black-helmeted, fool-choking Sith Lord.

A graveyard of old and rare planes that once were the glory of Persian aviation.

A fiery crater has been burning in the Karakum Desert since 1971.

Rusting ships sit in a desert where a sea used to be.

Once a bustling fishing port, this semi-ghost town is now more than 90 miles away from the shore.

Former island in the Aral Sea used to be a top-secret Russian bio-weapons facility.

World's oldest and largest space launch facility, once a secret missile testing site.