In the patio of the thermal baths of Iesso, the remains have been preserved of a large swimming pool, the natatio, measuring 9.35 by 6.30 metres and a maximum depth of 1.15 metres. Two sets of five stone ashlar steps enabled bathers to get in and out of the water comfortably. The bottom is paved with opus signinum with half piping on the edges, while the walls were clad with hydraulic cement.
The walls, of opus caementicium, were 1.30 metres thick and built on a foundation of large ashlars. Above, the edge of the pool was decorated with large stones.