These medieval tunnels provided water to the city above through centuries of civil war and plague.

At just 25 inches at its narrowest point, it's one of the tightest streets in the world.

The tiny Belgian town's ancient version of Mardi Gras involves eerie wax masks, sticks for warding off evil, and ostrich plume hats.

Dummies recreate radiological discoveries from history.

This bike path traverses the treetops.

The site where one of the most famous poems of World War I was composed is preserved to honor the legacy of the work.

One of the deepest indoor swimming pools in the world.

A marble Lucifer hides behind the pulpit of this Gothic cathedral.

A rare example of World War I trenches from the British front.

Meet the bronze beloved pet to a family of famous peeing statues.

From Tintin to the Smurfs, Europe's greatest graphic icons are celebrated in a grand Art Noveau gallery.

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

Climb inside an iron crystal magnified 165 billion times its normal size.

Lavish royal estate punctuated with heated greenhouses.

Each spring, a dreamlike carpet of bluebells overtakes the forest floor.

This platform is now a memorial dedicated to those who were deported during the Holocaust.

Dedicated to the European Sinti and Roma peoples murdered during the Holocaust.

The former Nazi concentration camp stands as a museum to remember this dark chapter in European history.

An underground city of salt.

Countless stories attempt to explain Poland's mysterious J-shaped trees.

At Chicago's revered Field Museum, visitors are invited to learn about and literally explore DNA.

The world's only known button museum.

This historic hotel invented the chocolate brownie.

Internationally known record store and purveyor of "dusties."