Millions of people visit Notre-Dame Cathedral every year.

Few realize that beneath the plaza in front of it lies evidence of a much older city.

As archaeologists excavate the site, they are uncovering artifacts and structures that predate the famous cathedral by centuries. Among the discoveries is a fourth-century coin from the era of the Roman emperor Constantine, who lived roughly 800 years before construction of Notre-Dame began in 1163.

The excavation is taking place on the Île de la Cité, the island in the Seine River where Paris first emerged. Layer by layer, archaeologists are uncovering traces of the people who lived, worked, and stored food there long before the cathedral's towers rose above the city.

Most visitors think of Notre-Dame as a monument to medieval Paris.

The ground beneath it tells a story that reaches much farther back in time.