The first Baths building (fig. 1) is dated the start of the II cent. A.D. under the Emperor Traiano, based on the building technique (opus quadratum) (fig. 2) and on type and distribution of environments.

Fig. 1 - The Baths I (from Bacco-Serra 1998, p. 1220. re-worked C.Tronchetti)

Fig. 2 - The imposing Baths I structure in opus quadratum (photo by Unicity S.p.A.).

The Baths are built because of the presence of a thermal hot water spring; accompanied by the cult of the Nymphs and of the healthy divinity Aesclepius from the start.

The central nucleus of the Baths is the large (12.20 x 6.10 m) pool (natatio, figs. 3-4) where the really hot water (54 degrees) was cooled with cold water for the curative bath.

Fig. 3 - The nucleus of Baths I: the natatio with the Nymphaeum in the foreground (photo by Unicity S.p.A.).

Fig. 4 - The natatio seen from the northern service environments (photo by Unicity S.p.A.).

On its long sides the pool had porticoed corridors, while on the short western one it had cisterns and service areas (fig. 5).

Fig. 5 - The service areas of Baths I (photo by Unicity S.p.A.)