This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

This picturesque castle in Poland has a rich history, including a legend that ties it to lost Incan treasures.

This World War II bunker now survives as a documentation of the Nazi regime and war crimes.

An eerie, derelict amusement park in Berlin.

This futuristic German spa allows visitors to float in a pool of salt water while zoning out to underwater techno.

There's a labyrinth of manmade tunnels stretching for over an acre beneath Bristol.

England's steepest street has such a drastic incline that locals tie their parked cars to lampposts when it gets icy.

A network of concealed passageways lies within a busy London Underground station.

The world's first underground farm is in a World War II bomb shelter beneath the streets of London.

This iconic 1,000-year-old medieval castle is a favorite of the film industry, most notably as the location for Hogwarts in Harry Potter.

A perilous medieval road leading right into the sea.

There's a labyrinth of medieval chambers hidden deep below a 15th-century pub.

It's believed King John of England stayed at this 13th-century castle before riding off to sign the Magna Carta.

Henry VIII's forgotten castle was built by the gunpowder-obsessed king to be defended solely by cannons.

The largest cave chamber in Britain—big enough to fit a whole cathedral inside.

This series of Scottish caves has no clear purpose other than baffling historians.

The latest in a series of long glass bridges suspended at terrifying heights.

An abandoned World War II tank left to rust in the downland.

The oldest family-run luncheonette in New York, last renovated in 1948, still serves food and drinks the old-fashioned way.

Eighteen miniature death-scene dioramas.

An Australian lake whose pink hue defies scientific explanation.

Abandoned railway line circling the city of Paris.

A toxic ghost town sitting atop a massive coal fire.

The unique waterfall in Iceland's south features a backdrop of geometric black lava columns.