The world's longest known cave system.

A seemingly desolate park that belongs to two countries actually supports all kinds of wildlife.

These wooden balconies that are both decorative and functional.

The largest salt farm in the Canary Islands is located at the foot of an active volcano.

The 20-mile causeway that separates the Netherlands from the sea.

Remarkably preserved body discovered in a peat bog.

This imposing tower named after the mythical sword of King Arthur is considered the tallest freestanding climbing wall in the world.

This still-celebrated home was a radical architectural experiment in 3D De Stijl.

Live out your childhood construction fantasies in this one-of-a-kind gyrating hotel.

The tallest waterfall in the Netherlands is a lovely, stepped feature that is only 50 feet tall.

A miniature city, once ruled by the Queen of the Netherlands.

This beautifully designed star-shaped fort was the premier defense system of the late 1500s.

The largest portal tomb in Europe has a capstone weighing around 150 tons.

This ancient Irish tower could easily have stood in for Rapunzel's prison.

Petrified footprints of one of the very first vertebrates to walk on land.

An Irish town has built a statue to a silent film actor simply thanks to the holidays he once spent there.

Ireland's most remarkable collection of these curious cone-shaped houses is worth the buzz.

One of the most popular megalithic sites in Ireland stands as a humbler version of Stonehenge.

This single room has survived the centuries with almost no deterioration.

The somber sculptures commemorating Irish Famine on the river Liffey in Dublin.

The gorgeous library at Trinity College is home to remarkable treasures of Irish identity.

A particularly disconcerting tribute to Irish martyrs.

One of Ireland's most infamous prisons for rebels including men, women and children.

The oldest inn in Kilkenny was established by the first convicted witch in Ireland.