An underground city of ancient quarry tunnels hid thousands of Allied soldiers in WWI.

Some locals believe the fabled lost gold of General Yamashida is hiding somewhere beneath the city.

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

The wartime boat shelter now doubles as an underground concert hall where musicians play on rafts along the waterway.

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

An abandoned canal route lies hidden under the streets of Manchester.

An eerie, little-known Neapolitan museum filled with preserved human remains.

This Neapolitan exhibition hall grants weird and wonderful insight into the intricacies of human biology.

The world's largest bioluminescent area was undiscovered by science until 2005.

Nature took her revenge on the "Wickedest City in the World."

Cavernous opening completely encased in Karst limestone.

The eerie remnants of an ancient forest were hidden under the sand for centuries.

An underwater statue of Christ blesses the depths off Key Largo.

The "worst battleship ever built" is enjoying a grand second act, submerged 30 feet in the Gulf of Mexico.

Though nicknamed the “Russian Freighter,” this ship was actually sunk off the coast of Pensacola by the U.S. government in a top-secret operation.

A "time traveler's" vision of human history stretches throughout Mexico City's Copilco metro station.

Ancient traditions and surreal birdwatching come into sharp relief along one of the world's most polluted rivers.

A health museum containing a 27’’½-foot intestine.

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

An unfinished rail tunnel that has since been used to house cheese and bats.

Rebuilt remains of a temple to Roman god Mithras.

Overshadowed by other, more famous Maltese catacombs, this ancient burial chamber has one of the best examples of a ritual agape table.

A centuries-old complex of hand-dug stone chambers has sheltered locals in times of trouble.

A labyrinth of ancient tunnels hidden below the city holds the ruins of 2,500 years of history.