Op-art architecture designed for happiness.

A botanically themed auditorium blooms within the world’s last operating double-decker theater.

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

A labyrinth hides in plain sight at each of the city's 270 Tube stations.

A musical composition 1000 years in length plays at the Trinity Buoy Wharf in London.

A clock designed to run with perfect accuracy for 10,000 years.

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

From lefty Jewish gathering site to hip cultural site in just 80 years.

A massive Montreal stadium that has seen nothing but trouble since its inception.

This collection of over 50 murals celebrates Brussels' rich history as the self-proclaimed home of the "comic strip."

This basement museum explores the art and artistry of fascism.

Where royalty waits in the height of luxury.

In the days before house numbers, these colorful signs held clues about various buildings' purposes.

This massive mural by famed pop-artist Keith Haring was hidden for almost 30 years until its 2018 rediscovery.

This pirate-broadcasting-tower-turned-restaurant played a rebellious role in the history of Dutch television.

Famed for its pie, this eatery has a hidden tunnel that allowed Catholics to secretly walk to church during the Reformation.

An ancient passage filled with stalls selling used books, once frequented by Vincent Van Gogh.

The world's first zoo for microbes collecting the horrible and wonderful things living on and around us.

An Amsterdam street lined with antique shops each one dedicated to a different obscure collection.

A floating cat sanctuary on an Amsterdam canal.

This cheese shop also houses a museum devoted to Dutch dairy.

Seven mini rowhouses fill the gap between 54 and 70 Westerstraat.

A curious collection dedicated to human and animal anatomy and pathology.

Cat art as far as the eye can see.