An overlooked and unassuming engineering marvel.

A 140-year-old castle with underground caves hidden in San Francisco.

A winding path built in secret on the edge of the continent.

A psychedelic labyrinth on the San Francisco bay.

An artsy hidden staircase leads to breathtaking views of San Francisco.

A theater sits abandoned on Roosevelt Road, once majestic in splendor, now majestic in decay.

The tragic reality of a town riddled with rumors of haunts, chemical spills, and serial killers.

The beautiful tunnel that ran from the lobby to Penn Station is still hidden underneath 34th Street.

The ghostly remains of an accidental sea.

This abandoned Navy base was transformed into an off-grid alternative living community for thousands of people.

A hand-built folk art installation covered with messages of God's love.

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

A 1,215-foot tunnel transports pedestrians beneath the River Thames.

The signs of a long buried road can still be found hidden beneath a London sewer grate.

A hands-off space for graffiti artists to hone and display their skills.

A kaleidoscopic warehouse-maze of handmade neon signs.

This churchyard arbor is surrounded by hundreds of gravestones placed there by author Thomas Hardy.

Hidden beneath the streets of London is a secret subway that only carried mail.

One of the few remaining casualties of the London Blitz, this destroyed church has become an enchanting public garden.

London's creepiest cemetery was once the site of dueling magicians and mobs of stake-carrying vampire hunters.

A Museum of Holography frozen in 1974.

A century-old network of freight tunnels underneath the city's historic business district.

What was once one of the country's largest confectionary plants is now an abandoned ruin, but is still pretty sweet.