During the Industrial Revolution this “Temple of Invention” was full of intricate miniature machines and gadgets.

A historic ruin just 20 feet away from a busy highway onramp.

A portal into the bricked up canal that runs through the heart of Washington D.C.

This small museum once served as a royal hunting pavilion for King Otto Friedrich Ludwig.

One lonely, modern pyramid has gone from corporate think tank to deserted boondoggle.

Though nicknamed the “Russian Freighter,” this ship was actually sunk off the coast of Pensacola by the U.S. government in a top-secret operation.

During World War II the Thorp building was guarded by the OSS, FBI, and US Navy because of the top-secret bombsight being produced there.

Where a guard killed nine German prisoners during one of the worst 20th-century POW camp massacres in U.S. history.

The haunting "Hell House Altar" is one of the few remnants of an abandoned college that has been a magnet for local legends.

A small museum in this border city displays the contraband art that has been stolen back from smugglers.

Thanks to its heroic service during WWII this Soviet castle has been turned into a monument towered over by a brutalist titan.

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

A marker honors the brief peace found by a tormented artist in the fictional land of Kcymaerxthaere.

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

Giant WWII fortresses that fired 8,000 rounds a minute, now home to thousands of pigeons.

Modern-day cave dwellers carve elaborate homes out of volcanic debris.

Salt cured mummies continue to turn up in this Iranian mine.

This Vietnamese hell cave lets visitors take a walk through a devilish underworld full of demons.

Underground hospital complex constructed by slave laborers during the German occupation of the Channel Islands.

This stunning concert hall has become a symbol of Iceland's national recovery.

One of the tallest buildings in Iceland, and perhaps, the most visually impressive.

Folklore and history carved into Brittany's coast.

This foolish Victorian sea fort was converted to a deluxe hotel.

A bloody history is captured in this former prison turned museum.