Paranormal investigators claim this abandoned asylum is the most haunted spot in the eastern U.S.

This abandoned orphanage-turned-asylum witnessed a terrible chapter in Québec's history.

Abandoned for over a century, this chilling train tunnel is now a destination for ghost hunters.

This abandoned hospital is one of the most haunted places in Costa Rica.

"Only the dead get off at Kymlinge."

For over a century, it was a crucial center of mental health treatment and also the source of many local legends.

An abandoned tuberculosis hospital said to be haunted by the patients who died within its walls.

The abandoned husk of a mental institution that failed to save a Jazz Age icon still sits nearly unchanged.

The underground mall's skylights look like they could be part of a spaceship.

This small but lovely sinkhole lake is the last remnant of a great prehistoric water basin.

The construction of a new church near Venosa was started in the 11th century, but never finished.

The steel cable lets adventurers zoom between two mountain towns at speeds of up to 75 miles per hour.

Crossing this suspension bridge feels like walking through the sky, a fitting tribute to the head of the Apollo program.

Ride a boat through this magnificent underground world, which now doubles as a theater.

These otherworldly caverns form Earth's oldest known cave system.

This Turkish cave boasts some rather bizarre associations with the King of Mount Olympus.

Rare recess caves fill this unbelievably beautiful Ohio park.

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

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

The oldest bar in Missouri is hidden within a limestone brewery cellar more than 50 feet underground.

This amazing ancient cave was kept secret for seven years to protect it from the public.

This Mexico City subway station will make you feel like you've been transported to the London Underground.

A labyrinth hides in plain sight at each of the city's 270 Tube stations.

A relic of Victorian grandeur hides beneath a busy South London road.