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

A section of the long chain that blocked enemy ships from entering Constantinople.

Legend says if you stick your thumb in the hole and it gets wet, your wish will be granted.

1513 Turkish world map, full of European state secrets and snarky commentary.

A marvel of Byzantine engineering under the streets of Istanbul.

The walls of America's oldest masonry fort famously "swallowed" cannonballs.

Arrr! The most authentic collection of pirate artifacts in the world... matey.

It's a pretty creepy lock-up.

St. Augustine's record-setting narrow street was designed to protect against pirates.

This communist behemoth is the heaviest building in the world and a legacy of a brutal regime.

This once-derelict building is now one of the most beautiful bookstores in the world.

The little-known second castle that William the Conqueror built in York.

This sculpture marks the spot where one of the greatest Roman rulers was proclaimed emperor.

Makers and sellers of the original and finest York Ghosts.

The final resting place of England's most notorious bandit.

One of the shortest streets in York has an especially odd name.

Follow the original medieval paths that wind through this intriguing section of the city.

The crypt of the imposing Stephensdom holds royal intestines and thousands of skeletons.

The beautiful baroque library of the Hapsburg empire.

A small museum filled with over 1,000 clocks, and one particular clock calibrated to run until the year 9999.

This London marker celebrates the spot where the world-famous English band was founded.

Behind one of the world's most impenetrable vaults is a massive collection of retail silver.

A mysterious shape catches the eye in this 16th-century painting.

The final resting place of a German diplomat's beloved companion.