Massive puppet-automaton machines created by French artist collective Machines de l’Ile.

A collection of outdoor mystical statues paying homage to indigenous Australians and ecological stewardship.

The great Danube river starts here.

The splendor of this 17th century golden chapel has led it to be called the "eighth wonder of the world."

The remains of what was, not so long ago, the world's largest city.

A marble Lucifer hides behind the pulpit of this Gothic cathedral.

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

17th century streets hidden under Edinburgh were once a breeding ground for the black death.

Underground hospital complex constructed by slave laborers during the German occupation of the Channel Islands.

The largest underground church in Europe.

Sparkling, calcite-covered skeletons of children who may have been sacrificed to appease an angry god.

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

Museum deep in the catacombs of St. Michael the Archangel Church is explored by touch, sound, and smell.

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

Descend into the tunnels to experience the life of a coal miner.

Beneath the city of Lourdes, France is a massive, concrete church.

Europe's second largest ossuary wasn't discovered until 2001.

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

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

The largest of London's subterranean rivers and once a mad, bad center of London life.

These remote Florida islands have a history of sea turtles and sunken treasures, and one of the world's largest coastal brick fortresses.

A gallery of underwater art that can be visited by man and fish alike.

Underwater art project will double as a massive artificial coral reef to help regenerate the damaged ecosystem.

A sunken idol welcomes parishioners to Davy Jones' Locker.