Found some 65 feet down near St. John in Lateran Basilica during the excavation of the new metro C line, the huge irrigation basin measures 115 feet by 230 feet.

“It’s so big that it goes beyond the perimeter of the metro work site. It has not been possible to uncover it completely,” Rossella Rea, the dig’s director, said at a news conference in Rome.

Dr Rea noted the basin was lined with hydraulic plaster and most likely extends, still preserved, beyond the work site toward the ancient city walls.

“On the basis of the size that had been determined so far, it could hold more than four million liters (1 million gallons) of water,” Rea said.

The massive basin was part of a farm dating to the third century B.C. In the first century A.D., the basin was added to existing structures, such as water wheels, used to lift and distribute the water along canals.

“Most likely it served as a water reservoir for crops as well as an area that made it possible to cope with overflows from the nearby river,” Rea said.

The excavation, carried out by archaeologists Francesca Montella and Simona Morretta, also brought to light various agricultural related items, such as a three-pronged iron pitchfork, and remains of storage baskets made from braided willow branches.

Lined up jars with their ends cut open were recycled as water conduits. Used tiles were also recycled to make canals. They were inscribed with the encircled initials “LT” — evidence that the farm belonged to a single owner.