Exploring the wilder, more passionate sibling of Erg Chebbi feels like discovering a secret sun-kissed planet from another galaxy.

The courtyard of this small Marinid madrasa is a dazzling example of intricate Islamic architecture.

One of the most spectacular canyons in the world.

The largest salt lake in the Sahara is mysteriously alluring.

Buildings from the fictional planet of Tatooine still litter the Tunisian desert.

Once a bustling fishing port, this semi-ghost town is now more than 90 miles away from the shore.

One of the most horrible legacies of the Cold War is a site where the Soviet Union tested nuclear bombs on civilians.

It's hard to believe the dizzying patterns covering this abandoned mine are completely natural.

Last remaining nuclear missile base in Ukraine.

The deepest metro station in the world.

A subterranean Holocaust refuge within one of the world's longest cave systems.

The largest catacomb system in the world.

The ghost town left by the worst nuclear disaster of all time is being taken over by nature and urban explorers.

This tunnel was built in order to protect the village of Daroca from flash floods during rainy seasons.

This former power station is now a museum dedicated to the region's coal-producing history.

An untouched lava tube accessible to the adventurers.

A road that winds through this natural cave is the only way to access the small, appropriately-named village of Cuevas.

The last of three massive blast furnaces that were once at the foundation of the Port of Sagunto economy.

An uninhabited village that preserves and documents rural architecture and an endangered way of life.

This magnificent market stands as a poignant symbol of the suffering caused by the Spanish Civil War.

Tour the inside of a massive lava tube formed during a 1949 volcanic eruption.

The largest medieval water tank on the Iberian Peninsula is preserved beneath the Cáceres Museum.

An out-of-place grove of sequoias thrives in northern Spain.

A curious carving hides on a marble column in the Mosque-Cathedral of Córdoba.