American and Egyptian archaeologists have discovered a rare structure called a nilometer in the ruins of the ancient city of Thmuis in Egypt’s Delta region. Likely constructed during the third century B.C., the nilometer was used for roughly a thousand years to calculate the water level of the river during the annual flooding of the Nile. Fewer than two dozen of the devices are known to exist.

“Without the river, there was no life in Egypt,” says University of Hawaii archaeologist Jay Silverstein, a member of the team who works at the site where the nilometer was found, near the modern city of El Mansoura. “We suspect it was originally located within a temple complex. They would’ve thought of the Nile