This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

The world's longest known cave system.

Most-visited and most-photographed slot canyon in the American Southwest.

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

In this historic region, homes and churches are carved into ancient volcanic rock formations.

The largest catacomb system in the world.

An Italian hillside ghost town.

Houses are tucked between, on, and underneath giant boulders.

Abandoned medieval hill town.

A Belgian self-built tower inspired by the Bible and ancient civilizations.

Siberian ghost town built by gulag prisoners.

A Spanish town built into the cliffs.

Southern Italy's spinning top village.

A dangling church built over a Colombian gorge.

This ghost town in Belgium will lose its street art when it ceases to exist.

Dramatic river bend surrounds a natural red-rocked pedestal.

The unique waterfall in Iceland's south features a backdrop of geometric black lava columns.

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

The celebrated eyesore created by starry-eyed sweethearts.

A series of waterfalls so massive they straddle the border of Argentina and Brazil.

An abandoned relic of the city's failed experiment with below ground transit.

"Respectful to the dead, safe and sanitary to the living, dedicated to the Genius of Memory..."

Once a farm where patients tilled the soil, this asylum lost its workforce when overmedicating became common practice.

Underground tunnels remain beneath this abandoned mental asylum.