A hidden tunnel leads to an abandoned slate quarry that's now a secret swimming spot guarded by cliffs.

The worst flood disaster in Welsh history might have been a lot worse if it had not happened on movie night.

One of the few surviving street funiculars in the world.

The only medieval bridge in Britain with its defensive tower still in tact.

A 19th-century funicular railway shuttles visitors up this surprising sea cliff to one of the world's largest camera obscuras.

A modest stone circle in North Wales.

The longest and tallest aqueduct in Britain can actually be sailed across.

The world's largest prehistoric copper mine had 30,000 animal bones scattered throughout the maze of passageways.

The highest peak in Wales is said to be the final resting place of a giant killed by King Arthur.

Three bridges straddle a stream, one stacked upon the other.

This awesome Spanish castle is so fantastical it was actually used on Game of Thrones.

A fiery-red region of scooped out land still largely untouched by civilization, tourists or otherwise.

This multi-level, aqueduct-footbridge hybrid is one of the greatest works of the Spanish Renaissance.

These magnificently sculpted tombs represent Spain's own legendary version of "Romeo and Juliet."

This amazing example of Valencian Art Nouveau looks more like a cathedral than a public market.

This small chapel is built of human bones and decorated with a golden skeleton.

An artificial date palm stands as a memorial to Warsaw's Jewish community.

On this Polish road, an optical illusion makes cars seem to roll uphill.

The giant statue and its model solar system honor the native Pole who declared that the Earth revolves around the Sun.

The Polish capital has had a deep connection to its guardian mermaid for almost a millennium now.

St. Mary's features one of the most spectacularly decorated cathedral interiors in all of Europe.

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