The maze-like transportation hub contains abandoned areas, derelict stores, and a resident bat colony.

The last remaining relic of San Francisco's glittering 1915 Panama Pacific Exposition.

This immigrant bridge once spanned the Thames but is now a fixture of an Arizona city.

Ninety feet off the valley floor is an ancient dwelling put under protection of the U.S. government by Teddy Roosevelt.

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

The highest point in Belgium is a staircase to nowhere.

The gilded, masterful astronomical clock will calculate time, dates, and planetary positions for thousands of years to come.

A radio telescope for listening to the stars, based in one of the cloudiest countries in the world.

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

This towering church is home to a world-famous organ and a treasure trove of medieval art.

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

The Bloemen Route, or “Flower Route”, stretches 25 miles through prime Dutch tulip country as a result of “tulip mania,” the world’s first economic bubble.

Aged nearly 1,700 years, the world's oldest bottle of wine is probably still drinkable.

A decomissioned facility where the German "Transrapid" maglev trains were once tested.

Once the longest swing bridge in Europe.

A unique bridge with an equally unique view.

Art historians have debated the identity of this medieval stone rider since the 19th century.

An imperial church bombed to ruins during WWII now stands as a jagged memorial.

These tenacious 14th-century towers defied demolition and are now a fairytale vacation rental.

A ruined castle stands atop a hill surrounded by dragon lore.

The castle where Martin Luther secretly translated the New Testament from Greek to German while in hiding.

In this brilliant blue spring in Germany, legend has it that a sad mermaid learned to finally laugh.

A monument to the three wars that led to German unification towers within a Berlin traffic circle.

The largest bell in the world has been swinging high above a Cologne cathedral for almost a century.