The watery entrance for condemned prisoners heading to the Tower of London is still visible along the Thames.

Pop culture has helped to keep this iconic British building from being destroyed.

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

This one has sailed five times as far as Sir Francis Drake's famous ship.

Hear the quietest sound from across the dome.

London's last remaining public soapbox site has seen speeches from Karl Marx, Vladmir Lenin, and George Orwell.

An unusual round church in London with a Templar past.

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

A horde of stray cats rule in this secluded Parisian cemetery under a bridge.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

This charming cobbled backstreet is the most Instagramable street in Paris, much to the residents' annoyance.

Remains of Roman amphitheater hidden in a sleepy quarter of Paris.

The celebrated eyesore created by starry-eyed sweethearts.

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

This former “ante-chamber to the guillotine” has been refurbished to illustrate the brutal history of French justice.

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

Op-art architecture designed for happiness.

Baroque celebration of the end of the Great Plague of Vienna.

After the famous composer died in Paris, his sister snuck his heart out of the country to honor his wish that it be buried in their native country.

A program of Communist glamour in post-Stalin Poland is preserved in this colorful museum.

On the Spree bank, discover one of the last reminders of a symbolic wall that divided, and was torn down by the people.

This water-tower-turned-high-end-apartment building was once the site of an early Nazi concentration camp.

Thousands of coffin-like pillars make up this controversial Holocaust memorial.

The bunker where Hitler took his own life is hidden under this deliberately ordinary parking lot.