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

This tri-part waterfall is a little bit of fairytale beauty in the heart of Luxembourg's Little Switzerland.

The very center of Paris is surrounded by a bevy of strange good luck rituals.

This massive library holds what was once the largest book collection in the world.

Abandoned railway line circling the city of Paris.

The oldest stone house in Paris was built by its most famous alchemist.

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

Paris' linguistic guide to the international language of, yes, love.

Notorious artist squat renovated into legal studios.

It's said this geological anomaly was caused when the Devil took a bite out of the Irish mountainside.

A massive Celtic cross secretly growing in an Irish forest.

A stunning Victorian abbey and walled garden on a lakefront.

This natural sea arch highlights Ireland's rugged coastal beauty.

Mortarless walls that stretch for miles are a lovely solution for creating grazing land in the harsh terrain of the Aran Islands.

A tribute to the incredible generosity the Choctaw Nation showed the Irish people during the Great Famine.

A moody and picturesque riverside ruin overgrown with vegetation.

An unusual manor of stunning beauty, born out of a crippling disease.

Obelisks and stone pineapples in the Irish countryside.

Local legend says the remains of Father Christmas lie within the medieval grounds.

This ancient Irish religious site is covered in ancient carvings, many of which have barely aged.

A tiny microvalley etched into the side of Knocknarea Mountain hides another world.

This Irish show cave is said to have been the site of a Viking massacre that claimed 1,000 lives.

The world of the ancient Celtic civilization explained through a bewitching 11 acre garden.

A nun spent 16 years painting the stunning Celtic mural that covers the interior of this WWI memorial.