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

The birthplace of Karl Marx is now a museum dedicated to the communist revolutionary's life and work.

An expansive complex of subterranean tunnels is all that remains of one of Europe's most formidable medieval fortresses.

This temple to the sturm and drang of Richard Wagner contains a dragon's cave and reptile zoo full of living "dragons."

It is said that each of the white stones in the red facade of this medieval house represents the head of a murdered Swedish noble.

The largest oak in England is said to have been the hideout of Robin Hood and his Merry Men.

Semi-precious Blue John stones have been mined in this cavern system for centuries.

A magnificent winding road through a steep valley, with deep natural caverns at the bottom

Some of England's last cave dwellers lived and worked within this huge cave.

This chain of artificial lakes was an ideal test ground for the aircraft that would carry the "bouncing bombs" used by the WWII "Dambusters" squad.

6000-year-old site covered in more than 50 megaliths.

A gorge full of caverns where early humans lived alongside cave hyenas, giant mammoths and wooly rhinoceroses.

A German apartment building hosting as many trees as human occupants.

This monument to those fallen in the World Wars is accompanied by a U-boat that Germany bought for a Deutsche Mark.

The last bastion of Roman presence north of the Alps looms over a German town.

Crop circles inspired this whimsical garden in Germany.

The birthplace of an eccentric alchemist and possible inspiration for the fictional reanimator of monsters.

Sitting in the basement of a castle, the largest wine barrel in the world is bigger than most apartments.

This historic funicular system combines the old with the new while offering delightful views over the surrounding valley.

It's good luck to be mocked by this monkey.

A preserved college lock-up that recalls a clubhouse approach to discipline.

Sweden's "good old days" painstakingly re-created in a model village.

The Swedish flag over this small naval fort signals that the nation is at peace.

It houses the remains of a 17th-century version of the Titanic.