The notorious "Hanoi Hilton" where Vietnamese colonial subjects and later American POWs, including John McCain, were imprisoned.

Life-sized painted illusions let pedestrians blend in with the street art.

Once the fallout shelter of a dictator, this sprawling bunker is now a combined history museum and contemporary art gallery.

The largest Roman amphitheater in the Balkans once held over 20,000 people, but managed to lay hidden for hundreds of years.

Invisible bridge parts the waters, invoking biblical images and preserving the ominous feel of an ancient moat.

A set of 27 bamboo tubes creates an eerie soundscape that changes with the wind.

A 1:20 scale piece of the Netherlands commissioned by a queen lets visitors take a stroll as giants.

Its town washed away centuries ago, this lonely tower by the sea tells the tale of a mermaid and a curse.

Watch the local Netherlands birds from inside this giant eco-egg.

A Portuguese Jewish cemetery filled with stunning grave monuments that date back to 1614.

This pirate-broadcasting-tower-turned-restaurant played a rebellious role in the history of Dutch television.

This striking steel sculpture transforms environmental data into art.

A remarkable medieval structure born from a 14th-century disaster.

This tiny island featuring 9th-century Viking ruins is only accessible at low tide.

A rocky sea serpent hides caves, blowholes, and a spectacular natural bridge.

Take a sea tractor to the Art Deco hotel on the island that inspired Agatha Christie.

A island reachable by a causeway at low tide is possibly one of the earliest Western European locations to be identified in text.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

This submersible road connecting a French island to the mainland is lined with safety towers for when the tide strands travelers.

Sometimes an island, sometimes a rocky outcropping connected to the world by a tiny suspension bridge.

A labyrinthine network of chambers with a storied past hides below Edinburgh's South Bridge.

Cold storage for agricultural biodiversity.

A market built atop boats is one of the few floating holiday markets in Europe.

Tables of secondhand books fill the inner courtyard of this 17th-century stock exchange.