<p>Plot of land in *Trans Tiberim, which belonged to L. Petilius and was situated immediately at the foot of the *Ianiculum (Solin. 1.21). The supposed ‘discovery’ of the stone sarcophagus and books of Numa Pompilius in 181 B.C. left the historical-collective memory of the spot in ancient literature (Liverani; Livy 40.29; Val. Max. 1.1.12). According to Cicero, the spot was not far from the <i>ara Fontis</i>, a famous altar to sacred springs on the Ianiculum (Cic., <i>Leg. Man.</i> 2.56; <i>OCD</i> 1436-37). The location of this altar is also controversial. A shrine dedicated to <i>Fons</i> in A.D. 70 was found in 1914 under the current Ministry of Public Education building on Via del Re (Mancini). While some scholars suggest that this must be a later monumentalization of the cult at the same spot (Aronen, Rodríguez Almeida), others are more critical of the discovery (Savage 30-31; Richardson 152-53). Richardson (4 and 152) suggests an alternative location for the <i>ara</i> and the nearby <i>ager</i> along the *Via Aurelia, especially where the street reached the lower stretches of the Ianiculum. Both hypotheses must remain conjectural in the current state of archaeological evidence; our map tentatively follows Richardson’s suggestion, which fits well with the ‘<i>sub Ianicolo</i>’ expression in the cited ancient sources as well as with the location of the springs in the region.</p>