Hidden in a grotto 26 feet below Nottingham's oldest hotel, this stylish subterranean drinking den is notoriously hard to find.

Live a night as a Byzantine King, with a minibar and private jacuzzi.

Hungarian mummies in beautiful painted coffins.

Once the home of a hermetic monk, this cave now houses a small church.

Human-made caves excavated to hide a secret society of wealthy pagans.

Discovered after draining a local pond, these ancient man-made caves are still a complete mystery.

Hidden within a Netherlands crater is a stone bed that lets those who lie in it see the sky as a curved surface.

Hidden away in a modern library are the remains of one of the 18th century's most celebrated wunderkammers.

On Japan's Tashirojima Island, cats are king.

19th century tunnels burrowed under Liverpool.

The world's largest collection of witchcraft regalia.

Museum in a swimming pool where the water still flows.

An ancient ossuary in Rouen.

Museum about life, death, and cannibalism in the Stone Age.

The oldest inn in England served brew to the crusaders.

The mysterious, fantastical woodland inspiration for "The Lord of the Rings."

A 200-year-old prison that now houses a peculiar crime museum.

Below a historic English market town lies a secret underground city complete with kitchens, laundries, storerooms, and an underground lake.

A house of bones — but nobody is quite sure to whom they belong.

Man-made cave with enigmatic carvings, some say made by the Knights Templar themselves.

The biggest and deepest Cold War bunker open to the public can be found in southeast England.

Shakespeare, David Bowie, Dr. Who, the Saxons, Druids and Romans are all claimed to have all made their mark on these massive man made tunnels.

Eclectic final resting place of the controversial explorer.

This shop and gallery is a modern-day cabinet of curiosities, unassumingly positioned along an urban thoroughfare.