There are hundreds of secret WWII bases hidden in British forests. Built to fight a Nazi invasion, they remained unknown for decades.

This 1960's Soviet submarine in Peenemünde Harbor claims to be the largest submarine museum in the world.

This lakeside bunker complex in Mamerki, Poland, housed the Supreme Military Command of Nazi Germany during World War II.

A fairytale Gothic castle steeped in Nazi treasure conspiracy theories.

Today this former Nazi concentration camp stands as a poignant memorial to the victims of fascism, and a warning from history.

The rusting hulk of a 250-foot-long merchant vessel remains where she grounded on the shore of Hudson Bay.

The haunting beauty of this little-visited wreck makes it a true hidden treasure.

From Austrians to Nazis to Soviets, this historic Budapest fortress has a history as complex as the city itself.

These crumbling Nazi artillery bunkers are now fair game for massive graffiti projects.

A line of disused military bunkers built to defend against Nazi invasion lies hidden on the outskirts of the city.

What was once an underground Nazi city is now Europe's largest bat reserve.

At night, this memorial illuminates a ghostly outline of a footbridge that connected two sections of the Warsaw Ghetto.

The site where a plot to assassinate Hitler was planned—and its conspirators were executed—is now a powerful memorial to the German resistance during World War II.

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

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

This open-air museum was originally a World War II testing ground for German missiles and rockets.

This sprawling museum holds one man's giant collection of real tanks.

The only surviving piece of a submarine said to be the most decorated vessel in U.S. history.

The superstar of Cold War nuclear bunkers has been featured in countless films but is now barely operational.

Twice a year, visitors can tour the desolate site that birthed the Atomic Age.

One of the largest fleets of armored vehicles in the world, sitting out in the California desert.

In the middle of an extremely unforgiving and desolate high desert is the most secret military facility in the world.

This deceptively beautiful island was once home to a German concentration camp and known as "Death Island."

This small Bavarian graveyard is filled with empty graves for German soldiers lost during the World Wars.