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

Sofia's legislative center is continuously reinventing itself.

A vast Communist-era exhibition space in the Bulgarian capital.

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.

"Holy Wisdom" overlooks the city that became its namesake.

A striking symbol of the Bulgarian capital.

This ancient theater, restored in the 20th century, lives on as a venue for dramatic and musical performances.

This massive stone and steel monument remembers a century-old battle that cost thousands of lives and nearly destroyed the city.

UNESCO-protected ancient burial ground.

This historic black and white monastery is one of Bulgaria's architectural jewels.

The spiral staircase hidden inside this giant Parisian column was likely built for Catherine de Medici's astrologer.

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

A Parisian exterminator's shop whose storefront displays stuffed rats hanging from traps.

The curious underground history of keeping Paris clean.

Egyptian Revival movie palace abandoned for decades and restored in 2013.

Statue of death tucked in the University of Medicine courtyard.

A scandalous Art Nouveau collaboration that set Paris all atwitter at the turn of the 20th century.

A horde of stray cats rule in this secluded Parisian cemetery under a bridge.

The oldest stone house in Paris was built by its most famous alchemist.

One of the oldest medical collections in Europe focusing on rare surgical instruments.

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

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

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