Experts reveal the sinister purpose of a strange bunker in Florida.

A castle was the site of horrific murders perpetrated by a countess.

Experts reveal the role towers in Greece played during violent feuds.

Experts reveal why a massive, alien-like fortress was built.

An abandoned prison has evidence of a chaotic riot.

An eerie set of buildings in Paris is evidence of an unspeakable evil.

A notorious dictator's paranoid fantasies created a strange bunker.

An underground labyrinth can warp the mind and turn friend into foe.

An abandoned theme park is home to secrets of a forgotten, imaginary world.

Experts reveal how Mafiosi was imprisoned on a Mediterranean island.

Test pilots and engineers embark on the first flight of the largest airplane ever built: the Stratolaunch, a one-of-a-kind aircraft designed to transport rockets, spacecraft and experimental hypersonic planes into the stratosphere.

Engineers in China are constructing the largest and most ambitious water transfer project in human history, defying nature to move vast quantities of water from the south to the dry north in Beijing, all without the use of pumping stations.

Explore three battlefield behemoths that command land, sea and air.

Inside access shows how an engineering marvel lifts boats to new heights.

The Pantheon is a 2,000-year-old temple with a dark secret. Today, using cutting-edge archaeological methods and brand-new evidence, experts look beneath the streets of Rome to find out why it was built and how it survived so long.

Experts investigate myths of a secret portal to hell in Hierapolis.