This ancient Greek place of assembly and marketplace is being revealed layer by layer below the modern Athens cityscape.

This legendary city has been a tourist attraction for centuries.

The hallucinogenic center of the ancient world.

The celebrated eyesore created by starry-eyed sweethearts.

What looks like a normal building is really a hollow facade hiding a subway vent.

A 16th-century fountain marks the site of a lost medieval cemetery once filled with mass graves.

The water-powered carriages are long gone, but for over a century this funicular railway has been the fastest route to the highest point in Paris.

In the best of all possible worlds, everyone will get a chance to see this statue in which Voltaire's heart is interred.

Medieval museum built over Roman baths that features famous tapestries of a lady and a unicorn.

A Parisian cemetery where the elite and artistic are interred beneath an eccentric collection of tombs.

A surrealist timepiece by the artist of melting clocks.

This charming cobbled backstreet is the most Instagramable street in Paris, much to the residents' annoyance.

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

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

This massive library holds what was once the largest book collection in the world.

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

This building has two facades of clashing architectural designs.

Said to be the only museum in the world dedicated to the most classic of all blooms, the rose.

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

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

Eastern Bloc nostalgia, anybody?

"Unity, Creativity, Beauty."

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

A striking symbol of the Bulgarian capital.