A marvel of Byzantine engineering under the streets of Istanbul.

Its sunny hues are believed to have inspired the town's color palette.

One of the city's most imposing Neoclassical buildings is also one of its most prestigious schools.

Iconic red and white plaques show the creative names invented to navigate the city streets.

If you can look past the nearby sewer pipes, this urban sinkhole grotto is a mini-paradise.

Clap your hands at the base of the pyramid, and the song of a sacred Mayan bird will echo through the air.

No telescopes, no power, no gift shop: The Observatory of Chichen Itza, one of the world's oldest observatories, takes astronomy back to basics.

Every equinox this Maya pyramid puts on a spooky ancient light show.

These azure waters were thought to lead to the Mayan Underworld.

In the middle of one of South America's most historic walled cities lies a bar with an enormous collection of Soviet memorabilia.

An inspired museum dedicated to the patron saint of enslaved people, inside the cloisters where he lived and died.

The caldera of this mini-volcano has been turned into a naturally heated mud bath.

This dizzying span over the ancient Algerian city was once the highest suspension bridge in the world.

This massive monument towers over the city as a constant reminder of the lives lost during Algeria's hard-won war for independence.

An extraordinary mix of Phoenician, Roman, early Christian, and Byzantine ruins beautifully set overlooking the Mediterranean.

The original resting place of Queen Cleopatra and Mark Antony's only daughter.

Founded in 670, this ancient mosque is an architectural time portal.

The ancient heart of Tunis is an enormous maze of narrow streets and alleyways.

An ancient burial ground believed to have been used for ritual sacrifice.

The best-preserved Roman small town in North Africa.

The largest and best-preserved Roman amphitheater in Africa was designed to seat 35,000 people.

The world's highest and most dangerous infinity pool lies at the precipice of Victoria Falls.

There is only one place in the world where the corners of four nations come together.

These secluded and abandoned anti-aircraft defenses have become one the most scenic viewpoints in Barcelona.