Gilmerton Cove, Edinburgh, Scotland. These tunnels below the city are said to have been built in the early 1700s by a determined blacksmith named George Paterson.

Rumor has it that some of Paterson's tunnels connect with tunnels once used by the Knights Templar — the most skilled soldiers of the Crusades.

But these tunnels aren't Edinburgh's only underground secret. Researchers believe that the Edinburgh we know today actually sits on top of an old Edinburgh, which was lost to the plague. When things got bad, city planners decided to start anew, building right on top of the forsaken ruins.

H.H. Holmes' Murder Hotel, Chicago, Illinois. One of America's most infamous serial killers was creepily efficient in his murderous techniques. To make sure things ran smoothly, he carried out the killings in a hotel of his own design, equipped with a network of gas chambers and body chutes. When guests checked into his World's Fair hotel, he would promptly gas them, sending their bodies down through the tunnels to his underground lair. This is where he would personally chop them up before selling their organs and bones.

Holmes' horrible hotel has since been demolished, only to have a post office built in its place. Employees are frightened by the mysterious tunnels below the building, which are lined with the very bricks Holmes used to build his house of doom.

Mao's Underground City, Beijing, China. Mao Zedong began building this 18-mile underground labyrinth below the city of Beijing as a shelter in the event of nuclear attack.

Though the tunnels are long-abandoned, they once held stores, restaurants, and even schools. It was designed to hold 40% of the city's population, but is now mainly traversed by tourists who have done their homework.

Predjama Castle, Slovenia. This 13th-century castle was built right into the entrance of Postojna cave, giving it an advantageous location in the event of an attack.

When the Hapsburgs came for Baron Erazem Jamski in the 15th century, for instance, the castle lord kept his family and servants hidden in the 3-mile tunnels. He would use the passageways to sneak out and retrieve supplies while the army struggled outside.

Moose Jaw, Saskatchewan, Canada. Tunnels were built underneath this town so that workers could still commute to work during the unforgiving Canadian winters.

But somehow the mob took control of these underground alleys, and during the prohibition, used them to hide and store alcohol. It's even been said that Al Capone had a hand in this activity, hence the tunnels' nickname, "The Chicago Connection."

The Winchester Mystery House, San Jose, California. In 1862, a woman named Sarah Winchester was hit with tragedy when both her six-month-old baby and her husband died within a few short years of each other. She consulted a psychic, who told her to move west and build a house so twisted that even the evil spirits cursing her would get confused.

And so the Winchester Mystery House was built. There are 13 wings in this mansion, each with a set of 13-panel windows and 13 chandeliers. The windy tunnels below the mansion can be accessed by several staircases with — you guessed it — 13 steps.

Dover Castle, England. Dover Castle has been around since before 1066 (when William the Conquerer made it his home), but it wasn't until the Napoleonic wars that they decided it was time to refortify and build some tunnels — a strategic move, considering that it served as the closest crossing-port from England to France over the English Channel.

The tunnels were used throughout history for different purposes. Most recently, they were prepped for the worst during both WWII and the Cold War, just in case nearby townspeople needed to evacuate.

The Magic Kingdom Tunnels, Buena Vista, Florida. Although it's not technically a real castle, the Magic Kingdom in Walt Disney World does boast a complex series of tunnels underneath the park.

Unfortunately, there's no Disney dungeon down there — just a series of pathways used by characters and cast members to quickly navigate each of the six lands that make up the Magic Kingdom.