A natural arch carved by a river gave its name to the so-called "Cave of Forgotten Dreams."

This is what trains will look like after the zombie apocalypse.

This historic island prison holds one of the dungeons from the Count of Monte Cristo even though it is a work of fiction.

An icy lake tucked among jaw-dropping alpine panoramas.

A subterranean shelter remains intact under a busy Paris railway station.

This massive library holds what was once the largest book collection in the world.

Abandoned railway line circling the city of Paris.

The macabre collection of Jacques Sirgent, an expert scholar on the undead and all of their trappings.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

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

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

A World War II torpedo launch station abandoned off the Polish coast.

This Nazi ruin was the site where Hitler was almost blown up by a suitcase bomb.

Launch site for nuclear war against Western Europe.

A narrow strip of land is home to the tallest migrating dunes in Europe.

A curious art park at the geographic center of Europe is home to 3,000 television sets and other wonders.

An old hill fort covered in over 100,000 crosses is a symbol of Lithuanian nationalism and beliefs.

Lithuania's outdoor sculpture trail of wooden folk art.

A former Latvian headquarters of the brutal KGB secret police is now open as a grim reminder of their atrocities.

An abandoned textile factory on the "island of crows" was home to Estonia's first workers' strike.

A top secret Cold War bunker hidden beneath a Latvian spa facility is now open for tours.

These crumbling Latvian forts make for some haunting seaside ruins.

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