UNESCO-protected ancient burial ground.

The remarkably well-preserved final resting place of a powerful ancient Thracian king.

A magnificent, glittering building tucked into a tiny Bulgarian mountain town.

A mountain pass in Bulgaria's Balkan Mountains was the site of four historic battles.

An abandoned saucer-shaped monument in the Bulgarian mountains.

Outstanding, eerie frescoes and decorations fill this 19th-century monastery.

An homage to the medieval rulers of Bulgaria.

The ceiling of Prohodna Cave in northern Bulgaria affords a peek at the sky via two deity-sized-eye-shaped holes.

Where Stoddart and Connolly suffered in the infamous Bug Pit.

This "tower of death" has acted as an observatory, a religious hub, and an executioner's lair.

Light still pours into the ruins of this ancient Islamic observatory.

The Avenue of Mausolea is a series of grand palatial tombs.

An abandoned 17th-century baroque hunting lodge with a wonderfully strange architectural plan.

These magnificent city walls rise from the barren sun-baked Spanish plains like something straight out of a fairytale.

This Industrial Age architectural and engineering gem helps visitors and residents alike traverse Lisbon, “The City of Seven Hills.”

A symbol of friendship between the mountainous regions of Switzerland and Bhutan.

For centuries no one knew there were hundreds of bones and precious artwork hidden beneath this Swiss church.

The ruins of an abbey believed to have been founded by Charlemagne in the 9th century.

This beautiful park on the outskirts of Rome protects the ruins of two colossal ancient aqueducts.

This tiny, tranquil Senegalese island serves as a powerful symbol of the Atlantic slave trade.

This park on the outskirts of Rome boasts two ancient Roman aqueducts and a medieval tower.

A rather vulgar example of the transition from Latin to the Italian vernacular.

The best preserved Paleochristian basilica in Rome hides a number of oddities and secrets.

Dakar is home to one of the oldest African art institutions in West Africa.