There’s a secret bunker inside the Brooklyn Bridge, but the city won’t say where.

This abandoned dream castle and its cemetery look out over the city that once inspired their creation.

High in the clouds of San Marino sits an ancient tower with some curious prisoner graffiti.

These former Cold War launch sites have been preserved for tourists to see where the button might have been pushed.

These ruins in Volvi were once at the intersection of the Eastern and Western Worlds.

One of the few standing Roman lighthouses and a labyrinth of tunnels are a couple of the secrets at this historic castle.

This medieval Estonian tower provides access to a series of tunnels that have been home to soldiers, punks, and fine art.

This Italian foot path winds through the mountains via 52 individual tunnels.

Running under an entire mountain, the longest highway tunnel in North America is kept fresh using portal fans and jet fans.

This road in the Great Smoky Mountains was supposed to assuage a displaced community, but ended up a $52 million dead end.

The prototype for all Nazi concentration camps.

The prison where Hitler wrote "Mein Kampf," and 278 Nazis were later executed for war crimes.

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

The only remaining U-boat that is still floating never actually saw any action in World War II.

An immersive environment that is part haunted house, part choose-your-own-adventure, and part jungle gym.

The site of a concentration camp where inmates were made to take part in one of the largest counterfeiting operations in history is now hauntingly empty.

Serbia's master of striking memorials erected a forgotten, hidden gem in the hills outside a tiny village.

An American city stranded at the tip of a Canadian peninsula where strict adherence to the "49th parallel rule" became problematic.

A dilapidated dock tells the story of Titanic's missed arrival.

Great place to have a drink and a bite to eat in an atmosphere laden with New York's Bohemian past.

Elevated freight railway turned wildly successful urban park.

A ghost town that held steady longer than most, but finally gave up the ghost.

Raging waters have created a strange geological site that looks like something off of a progressive rock album cover.

The largest permanent Monopoly board makes one of the longest games even bigger.