The nymphaeum of Egeria (Ægeria) was probably part of the waterworks of a nearby villa belonging to to Herodes Atticus, whose famous triopion, an agricultual estate dedicated to the memory of his wife Annia Regilla, extended from the Appian Way to the banks of the Almo river (currently named Almone).

The mistaken identification with the Grotto of the Nymph Egeria, mentioned in the ancient sources, dates bak to the period of Renaissance humanism when a number of scholars came to believe that the nymphaeum was the location of the spring and sacred woods where according to the tradition Numa Pompilius (second king of Rome) met with Nymph Egeria.

circa 150 CE

The nymphaeum, which had been entirely built above the ground by cutting in to the natural bed, and subsequently covering it with earth, was separated from the main body of the residential villa and probably was used for short stays away from the summer heat.

Recent archaeological investigations have identified at least two main building phases, one dating back to the second century CE, when the current nymphaeum sturucture was built to replace a previous building of unknown purpose, and a second one concerning the renovation of the building under emperor Maxentius (reigned 306-312 CE).

The original structure, built in Opus Mixtum (a tufa opus reticulatum masonry framed by strips of brickwork) was divided in to two main spaces. The first, in which the water gushed from a masonry conduit embedded in to the natural bed, is a rectangular room covered with a barrel vault, with semicircular niches which hosted statues and a masony arch decorating the back wall where a headless reclining semi-nude male-figure statue representing probably the god Almo stands. A channel carrying mineral water from the nearby springs under the Via Appia still feeds around the back of the building just below the statue. A sort of avant-corps divided in to two symmetrical wings covered by barrel vaults and divided by a central space opening on to the valley, was built perpendicularly to the said room.

In the fourth century CE, the complex was extensively renovated, as evident by the presence of Opus Vittatum masonry (strips of tufa blocks alternating with strips of brickworks), a technique also found in the nearby complexofthe Villa of Maxentius. In fact, when Herodes Atticus died, his villa, as well as the nymphaeum, became the emperor's property.

During the Middle Ages the monument was abandoned, and remained as such until it was rediscovered in early 1500s when the building was started to be studied and reproduced in drawings and prints by the Renaissance era architects such as Baldassare Peruzzi and Antonio da Sangallo the younger. The first archaeological excavations in the nymphaeum area would however be carried out by Fea only as late as the early nineteenth century CE.