A memorial to the nearly 10,000 American soldiers laid to rest on the beaches of Normandy during WWII.

The haunting concrete remains of a Nazi pebble crusher can still be found in this small French commune.

A massive, secret marina for the Nazis' most formidable weapon in the Atlantic.

An underground city of ancient quarry tunnels hid thousands of Allied soldiers in WWI.

The subterranean "lake" below the Paris opera house inspired the Phantom of the Opera's lair.

This futuristic observatory at the top of the Pyrénées offers unrivaled views.

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

High atop the Eiffel Tower is a small apartment built exclusively to entertain the science elite and make the rest of Paris jealous.

Paris' linguistic guide to the international language of, yes, love.

Abandoned railway line circling the city of Paris.

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

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

This unassuming museum's slogan translates to “there is a limit of sadness, anxiety has no limits."

The largest catacomb system in the world.

A former top secret nuclear fallout shelter now operates like a hotel stuck in the Cold War.

The largest manmade caves in Europe now function as a motorbike club's headquarters.

Underneath the city that invented pilsners are a series of historic tunnels that have been storing beer for centuries.

Two monstrous faces loom over a village in Czechia.

Europe's second largest ossuary wasn't discovered until 2001.

Underneath this small church are 15,000 bones, arranged by an obsessive academic.

There are two dozen tanks just sitting in an open field in Lower Saxony.