Five sultans and their families rest in eternal splendor, teeming with overlooked beauty and drama.

A marvel of Byzantine engineering under the streets of Istanbul.

The deepest underground city in Turkey was designed to protect 20,000 people behind massive stone doors.

One of the largest underground systems runs under Canada's largest urban center.

One of the Seven Wonders of the Modern World holds the world's tallest metal staircase.

A collection of more than 12,000 shoes, displayed in a building shaped like a shoebox.

A 98-room castle in the middle of Toronto that bankrupted a 19th century electricity multimillionaire.

Lost subway station nestled beneath Toronto's busy commuter line.

An unusual round church in London with a Templar past.

The hallucinogenic center of the ancient world.

Construction of Barcelona's iconic (but controversial) church is expected to be completed in 2026—a century after the death of its architect.

Kings Cross Station pays tribute to its role in Britain's best-selling book series.

France's most famous cemetery, with some of its most curious tombs.

In this historic region, homes and churches are carved into ancient volcanic rock formations.

Famously misunderstood wonder of the ancient world

This iconic Parisian bookstore doubles as the "Tumbleweed Hotel" for traveling writers.

The vast, legendary catacombs hold secrets much stranger than stacked bones.