The ruins stand as a somber reminder of France’s worst civil catastrophe of the 20th century.

This massive blockhead sculpture hides an entire library.

This mosque was based on Sub-Saharan Islamic architecture and created for the Senegalese Tirailleurs.

A medieval military street hidden beneath a charming French fishing village.

Oliver Cromwell marched his army over this narrow bridge to victory at the Battle of Preston in 1648.

A harrowing sculpture commemorates the four people shot down during the workers strike of 1842.

Locals spent years successfully fighting to save this beloved Brutalist behemoth from demolition.

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

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

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

In this cobwebbed old operating theater, you can practically hear the screams of the unanesthetized patients.

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

Several plaster noses are hidden in plain sight around London's Soho neighborhood.

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

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

A hands-off space for graffiti artists to hone and display their skills.

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

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

An easily overlooked stretch of ornate window fronts and gaslight lamps that could be right from the pages of Dickens.

Banksy gives themself artistic license with this piece of London street art.

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

Hidden beneath the streets of London is a secret subway that only carried mail.

This churchyard arbor is surrounded by hundreds of gravestones placed there by author Thomas Hardy.

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