The natatio, the large pool, is the heart of the Baths I complex (fig. 1). 

Fig. 1 - Map of Baths I. In green the natatio (from Bacco-Serra 1998; re-worked by C. Tronchetti).

Measuring 11.20 x 6.10 x 1.60 m in depth, it is surrounded by access steps (fig. 2). 

Fig. 2 - The natatio seen from the East (photo by Unicity S.p.A.).

The long sides and the western one have four steps, while there are five on the eastern side.

The hot and cold water mixed flowed in on the western side where it streamed from a mouth in the shape of an animal protome, now no longer present (fig. 3). The service areas and water mixing tanks are also on the west side (fig. 4)

There is no way of knowing for sure whether the natatio had skylights in its barrel-vaulted roof like the portico, but it is concretely possible.

Fig. 3 - View of the natatio with the water supply point (photo by Unicity S.p.A.).

Fig. 4 - The natatio with its service areas in the background (photo by Unicity S.p.A.).

However, we certainly know that the pool was maintained continual over time: this is proved by the numerous votive inscription fragments reused at later dates to replace damaged steps (fig. 5).

Fig. 5 - Inscriptions reused as steps in the natatio in the Late-Antiquity period (photo by Unicity S.p.A.).