This French "sea of rocks" is packed with stunning rock formations and its own medieval castle.

Massive puppet-automaton machines created by French artist collective Machines de l’Ile.

America's only nuclear missile silo open to the public.

This top-secret nuclear bunker hidden in a Bosnian mountain has been converted to a modern art exhibition space.

This Cold War–era fallout shelter was actually a storage facility for the nation's emergency currency.

This bike path traverses the treetops.

Each spring, a dreamlike carpet of bluebells overtakes the forest floor.

Climb inside an iron crystal magnified 165 billion times its normal size.

This recently restored crypt was once plagued by liquefying coffins and exploding caskets.

An open-air sewer system that was covered piece-by-piece over 300 years.

A research lab and hands-on museum where visitors can experiment with retro technology.

This huge metal monument is slowly being stripped to its concrete skeleton.

This stunning, out-of-the-way Croatian monument commemorates a community's uprising against occupying fascist forces.

Repurposed space and ultra-modern construction collide to create this geometric wooden spectacle.

This bike path was sunk below water level for maximum wonder.

The "stairway to heaven" lookout tower was built to resist weather, arson, and the laws of physics.

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

An homage to the humble watering can.

This peculiar museum offers hands-on interaction with seemingly intangible principles of mathematics.

A German apartment building hosting as many trees as human occupants.

This German bone house contains the remains of some 20,000 souls.

A scale rocket of an Ariane 5 rocket serves as a monument to a pioneer of space science.

A unique collection of electric insulators on display in a former transformer station.

This impressive castle looms over small German town.