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

A subterranean ice warehouse that has lain hidden beneath central London for over a century is now open to the public.

The curious underground history of keeping Paris clean.

Deep under Manhattan is the largest construction project in city history. So far, the project has taken the lives of 23 workers, or "a man a mile."

In the middle of downtown Brooklyn lies a 165 year old secret hidden right below the surface (closed to the public indefinitely).

The world's largest drain sits below the city of Kasukabe, Japan.

A natural resemblance of the monster of film lore.

America's post-nuclear-attack chambers of Congress.

Measuring approximately 200 feet long, Swampy is the world's largest alligator-shaped building.

These 100-year-old tunnels in downtown Provo house artists, restaurants, and a few surprises.

Beneath the southern end of the Golden Gate Bridge is the “Gibraltar of the West Coast,” a fort built to protect the San Francisco Bay from naval attack.

The remains of the iconic train crash from the movie The Fugitive can still be found rusting along the Great Smoky Railroad.

Visit an imposing 13th-century Transylvanian citadel lurking not far from the region's more heavily touristed attractions.

A 12th-century castle featuring a sensory path where partially-sighted visitors can experience regional history through touch and sound.

In the heart of Brigham Young's promised land sits a festering plot of industrial ruin that has been claimed by society's outcasts.

An intricate network of underground tombs carved into volcanic rock.

A tunnel that brings more than a century of history to life from the deepest parts of the Niagara Gorge.

One of Antoni Gaudí's most classic buildings is well-known for its "dragon-back" design.

This mural changes every few years.

This 400-year-old sweet shop serves a delicacy some have described as "edible incense."

For more than two centuries, this family-run establishment has been serving up some of Ireland's best oysters.

This Birnam-based garden honors the legacy of 19th-century children’s book author Beatrix Potter.

Monty Python fans will recognize this abandoned mine as the Cave of the Killer Rabbit of Caerbannog.

London's smallest police station is barely the size of a phone booth.