Ancient paintings known as the "Sistine Chapel of Cave Art."

Sacked by Vikings, blessed by William the Conqueror, dealt a fatal blow during the French Revolution—these ruins have seen it all.

Indents in the pavement are all that remain from a grisly death machine in Paris.

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

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

A scandalous Art Nouveau collaboration that set Paris all atwitter at the turn of the 20th century.

High atop the Eiffel Tower is a small apartment built exclusively to entertain the science elite and make the rest of Paris jealous.

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

A garden of flying flowers.

The patron of this extravagant Art Nouveau confection insisted that nothing less than a boat should project from its facade.

The Belgian royal family's private depot now stands abandoned and forgotten alongside the tracks.

A fantastical 13th-century fortification sandwiched by a modern hotel in central Brussels.

The "stairway to heaven" lookout tower was built to resist weather, arson, and the laws of physics.

This Greek temple, open only an hour a day, was the site of a war of tastes between an architect and an artist.

A Belgian self-built tower inspired by the Bible and ancient civilizations.

Hiding the longest subterranean river known in the world.

Part modern art project, part traditional chapel, this see-through church is only substantial from the right angle.

A failed attempt to control drifting sands yielded a gnarled sylvan wonderland.

The northernmost point in Denmark is a turbulent sandbar created by two clashing seas.

Preserved "bog person" that is over 2,000 years old.

The de facto symbol of Copenhagen is this bronze fairytale which cannot seem to keep its head for very long.

A walkable map of the world, made from soil and stone by one man.

Nordic giants resurrected from the sea and restored to their former greatness.

A small Denmark town displays a pair of titanic jaw bones as their community symbol.