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

Under a glass sidewalk lie the remains of some of Manhattan's oldest buildings.

New York City's smallest piece of private property.

A heavenly experience, right here on Terra Firma.

The beautiful baroque library of the Hapsburg empire.

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

A rare view of a preserved Victorian science museum.

Ruins of a German Castle on top of a towering stone pillar.

Construction of Barcelona's iconic (but controversial) church is expected to be completed in 2026—a century after the death of its architect.

A tiny theater hidden in an alley plays a constant stream of short silent films for anyone willing to peer through the peephole.

Northern Ireland's Brobdingnagian stepping stones.

The most iconic structure in Scotland has been home to bishops, colonels, and Sean Connery characters.

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.

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

The birthplace of an eccentric alchemist and possible inspiration for the fictional reanimator of monsters.

Enormous lake at the bottom of a unique cave system.

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

Famously misunderstood wonder of the ancient world

The haunting "Hell House Altar" is one of the few remnants of an abandoned college that has been a magnet for local legends.

The ghost town left by the worst nuclear disaster of all time is being taken over by nature and urban explorers.

An ancient capital city that dates back 3,000 years old and harbors many surprises.

The unique waterfall in Iceland's south features a backdrop of geometric black lava columns.

A church of bones, decorated with 40,000 human skeletons.

The fairy tale castle of the "fairy-tale king."