Formerly Yugoslavia's "forbidden island," this sleepy little city still holds the abandoned remains of military strongholds.

When Croatia became independent of Yugoslavia, the splintering governments left behind these concrete tunnels.

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

A chain of 16 wondrous lakes formed as a result of the gradual dissolution of soluble layers of bedrock.

The oldest church in Iceland is one of the few to feature ornate decoration.

A trio of hidden hot tubs takes advantage of Iceland's natural geothermal vents.

Climb right into the heart of this colorful Iceland volcano.

Iceland's "Smoky Valley" is a beautiful river that is shrouded in a constant cover of steam clouds.

This fantastical waterfall is hidden in a cave located in a cracked cliff.

This volcanic cave lake is essentially hard proof that Iceland is the most magical place on Earth.

Rising from the sea like a stone monster.

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

A unique museum tells the story of the day-to-day life of World War I soldiers.

This small commune with an even smaller name appears to have an unlikely amount of dead ancestors.

A memorial to the nearly 10,000 American soldiers laid to rest on the beaches of Normandy during WWII.

The haunting concrete remains of a Nazi pebble crusher can still be found in this small French commune.

A massive, secret marina for the Nazis' most formidable weapon in the Atlantic.

An underground city of ancient quarry tunnels hid thousands of Allied soldiers in WWI.

The subterranean "lake" below the Paris opera house inspired the Phantom of the Opera's lair.

This futuristic observatory at the top of the Pyrénées offers unrivaled views.

A subterranean shelter remains intact under a busy Paris railway station.

High atop the Eiffel Tower is a small apartment built exclusively to entertain the science elite and make the rest of Paris jealous.

Paris' linguistic guide to the international language of, yes, love.

Abandoned railway line circling the city of Paris.