This extensive collection covers thousands of years of Polish military history.

A bronze replica of an elm tree that honors the victims that perished in Pawaik Prison.

This hidden green space is the city's second largest conservatory.

The signs of a long buried road can still be found hidden beneath a London sewer grate.

Wedged between two buildings at the Great Ormond Street Hospital is this secret music installation.

A historic London pub with prison cells in the cellar and a "spy room" on the second floor.

A makeshift shrine to the late rock star is still kept up on the nondescript façade outside 1 Logan Place.

The curios of pharmacist and collector Henry Wellcome.

The world's first underground farm is in a World War II bomb shelter beneath the streets of London.

The quaint little store that is said to have inspired a famous Dickens novel was only given its name after the book was released.

This 1920s Dutch barge is now a floating bookstore.

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

This South London cemetery holds the remains of more than 15,000 people.

A tongue-in-cheek arcade where the games are based on everyday life.

Winding paths and ramshackle mausoleums in the oldest "park cemetery" in London.

The perfectly preserved underground rooms where Churchill plotted the war against Germany.

This shop and gallery is a modern-day cabinet of curiosities, unassumingly positioned along an urban thoroughfare.

A morbid secret lies hidden within the beautiful walls of the Boston Athenaeum.

The home of aviation trailblazers, in a remote corner of the big city.

Brooklyn's Barkaloo Cemetery is the resting place of a scant two bodies.

Historic Greenpoint theater, turned roller disco, now housing a chain pharmacy store.

Once a popular watering hole among longshoremen and shipyard workers, this neighborhood treasure spans more than 100 years of Red Hook history.

Waterway populated with abandoned projects, including a submarine.

A no-frills cinephile haven known for showing offbeat, underseen films.