The former locus of Boston's high culture has been shuttered and buried for over 70 years.

NYC's legendary underground network for bovine transportation.

Walk over a 150 year old manhole.

The world's toughest modern Christian pilgrimage leads directly to the gates of Hell.

Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

Beijing's expansive subterranean hiding spot now crumbles away as a tourist destination.

A former copper mine which became a truly unsuccessful prison.

The world's largest man-made underground caverns offer unparalleled underwater wonders.

One of the most remarkable caves in all of Iceland was created by a once raging river of lava.

This famous Nevada gold mine, once a hotbed of Wild West shenanigans, is now open for tours.

This former limestone quarry is now a subterranean wonderland of grand stonework.

The labyrinthine storm tunnels beneath the city of Melbourne are ruled by a band of urban explorers.

This series of underground chambers contains Europe's largest subterranean lake that is slowly drained each day.

To literally keep their waste out of their own backyards, the citizens of this English city built an impressive sewer system that is still in use today.

These sub-farmland caves are home to a majestic stalactite known as the "Crystal King."

This subterranean necropolis holds three floors of antler-dug grave caves.

The superstar of Cold War nuclear bunkers has been featured in countless films but is now barely operational.

This ancient subterranean megalopolis is known for the hot oil traps that were installed above each of its entrances.

The deepest underground city in Turkey was designed to protect 20,000 people behind massive stone doors.

While not Turkey's deepest underground city, this subterranean redoubt is the largest.

The second largest in the world, this massive underground void has its own subway.

Discovered by cows, these upstate New York show caves are presented with a healthy dose of psychedelic folk art.

The hidden "ballroom" under a mansion lake was built by a criminal who ate a cyanide pill during his trial.

The ruins of an ancient island city built atop a coral reef in the South Pacific are spectacular and strange enough to have inspired H.P. Lovecraft.