The U.K.'s first underground railway line is now a lively thoroughfare for runners and cyclists.

A labyrinthine network of chambers with a storied past hides below Edinburgh's South Bridge.

In a 345-million-year-old cave that's 200 feet underground, dinner awaits.

This Old West prison built into the side of a cliff was said to be inescapable.

The site where a nuclear bomb was lowered over 8,000 feet underground and detonated as an experiment in natural gas extraction.

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

The entrance to this sinisterly named tunnel hides behind a Tim Hortons.

WWII-era bunkers are hidden under the beaches of this picturesque park, which is still scattered with wartime watchtowers. .

Just your average Marie Antoinette–themed speakeasy in an abandoned 19th-century subway tunnel.

Canada's oldest railway tunnel is illuminated in a rainbow of color as it passes under the streets of this small city.

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

This old cement mine has been used as everything from a mushroom farm to a recording studio.

A once grand hotel is being retaken by the wilderness it once capitalized on.

An abandoned castle owned and ignored by Masons carries a cursed history.

These World War I shooting range tunnels are now haunted halls of underground graffiti accessed through the roots of a tree.

Over 100 concrete munitions bunkers are all that remain of a Pennsylvanian village seized by the US government.

This New York estate was designed to resemble a medieval Scottish castle.

Eclectic architecture in the woods of New York.

Opened in 1947, this is the oldest continuously-operating drive-in theater in New York.

An abandoned asylum where patients have been forgotten but their possessions remain.