The largest free-standing aquarium in the world resides in a hotel lobby in Berlin.

The site of a concentration camp where inmates were made to take part in one of the largest counterfeiting operations in history is now hauntingly empty.

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

This historic stone bridge has an entire neighborhood clinging to its sides.

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

The last bastion of Roman presence north of the Alps looms over a German town.

Sitting in the basement of a castle, the largest wine barrel in the world is bigger than most apartments.

A preserved college lock-up that recalls a clubhouse approach to discipline.

It's good luck to be mocked by this monkey.

Dress up like a miner and take a slide into the disco-lit depths of this once foundational salt mine.

The prototype for all Nazi concentration camps.

Go surfing on this urban wave, "hacked" by local surfers.

This beautiful 19th-century temple stands atop the ruins of its predecessor.

Thousands of tourists and residents in Cologne affix "love padlocks" on the Hohenzollernbrücke Bridge to symbolize their affections.

Birthplace of the "Cat in the Cradle" and a cluster of iconic, original windmills that let the Dutch tame the seas.

This small town in the Netherlands has no roads but instead, miles of canals and over 100 bridges.

A uniquely preserved neo-Gothic church that is now home to a collection of art and woodcarvings.

The attic of a four-century-old canal house hides a clandestine church.

Amsterdam's former hotspot for public executions and dissections.

A floating cat sanctuary on an Amsterdam canal.

Colorful and chaotic stacked hotel in North Holland.

Unsurprisingly, Holland is home to one of the most magnificent flower gardens in the world.

This 11th-century castle sits so low you can't see it behind the surrounding buildings.

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