Longest fortress system anywhere in Europe.

This Croatian fairytale castle is slowly falling apart thanks to years of neglect.

Large, airy hallways hide within an impenetrable fortress that beckons urban explorers.

One of New York's best collections of pinball is hidden in a secret bar in the back of this laundromat.

One of the world's tallest waterfalls wasn't revealed to the world at large until 2006.

An Archaeological Wonder of Peru, and the largest stone ruin site in the New World.

This South London cemetery holds the remains of more than 15,000 people.

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.

A subterranean ice warehouse that has lain hidden beneath central London for over a century is now open to the public.

The perfectly preserved underground rooms where Churchill plotted the war against Germany.

An incarceration museum on the spot of possibly the oldest prison in England.

In this cobwebbed old operating theater, you can practically hear the screams of the unanesthetized patients.

Since 1874, this macabre collection has developed into a museum devoted to Scotland Yard's mementos of dastardly deeds.

Walk over a 150 year old manhole.

An uber-secure, windowless tower of doom in the center of Manhattan is an NSA spyscraper.

Amid the bustle and noise of the Financial District hides a secluded garden oasis above the city streets.

This Wall Street bar was once a hub of revolutionary activity where Washington famously bid farewell to his troops.

A clock set into the concrete outside a Manhattan jeweler has been telling time underfoot for over a century.

A beautiful and abandoned New York subway station from 1904, complete with chandelier.

The oldest house in Manhattan is now a museum that remembers the home's sordid history of scandals, vice-presidents, and ghosts.

This abandoned fort is said to be the most haunted place in India, so much so that entry is prohibited after sunset.

The final resting place of New Orleans' most famous voodoo priestess.

The oldest cemetery in New Orleans, resting peacefully for over 200 years now.

A 200-year-old bar in the historic French Quarter refuses to give up its place in history, nor its role in securing ours.