A volcanic tunnel illuminated by natural skylights offers a shaded respite from the desert sun.

Caves famous for the dragon-slaying saint who lived there.

These towering spires and glowing walls are carefully crafted from hundreds of thousands of icicles.

This amazing pedestrian suspension bridge in the Swiss Alps would give Indiana Jones nightmares.

Hundreds of stone stairs and a winding medieval bridge connect this haunting Spanish island to the mainland.

These ancient sunken lanes were naturally tunneled into the soft ground by countless footsteps.

Hidden in a secluded Greek cove is a pristine beach marred only by the rusting smuggler's hulk stranded on its shores.

Gnarled canopies of pink and purple wildflowers arch over the trail to the summit.

The tunnel's elegant proportions keep you snug with no claustrophobia freak-out.

This valley of hot springs and boiling mud pots is home to the record-holding "Geysir," which originated the English word.

A living archive of urban activism and space reclaimed.

These curious caverns have been home to mushrooms, gangsters, and disco.

An underground city of salt.

Though strikingly beautiful, Seattle's other railway station is now largely forgotten.

A Victorian Gothic masterpiece housing a world-class collection of rare books and manuscripts.

A marvel of Byzantine engineering under the streets of Istanbul.

A sculpture for the sun and stars, created by Nancy Holt.

An overlooked and unassuming engineering marvel.

The hotel where Martin Luther King Jr. was shot is now a museum dedicated to his work.

Following the trail of an old metro line, this scenic path is haunted by a creepy spriggan statue.

The largest cave chamber in Britain—big enough to fit a whole cathedral inside.

This enchanting 600-year-old tree is so magical it inspired the beloved tale of Pinocchio.

This 170-year-old Swiss guesthouse built into the side of a mountain is best accessed by a cable car.

The oldest pub in Ireland may also be the oldest surviving bar in the entire world.