<p>A natural spring (Ov., <i>Fast.</i> 1.708: <i>Iuturnae ... lacus</i>; Val. Max. 1.8.1c Shackleton Bailey: <i>apud lacum Iuturnae</i>; Front., <i>Aq.</i> 4.2: <i>fons</i>) in the SE corner of the *Forum, where excavations in 1900 revealed a marble-lined basin <i>c</i>. 2 m deep and <i>c</i>. 10 x 10 m at the top, aligned with the Temple of *Castor. It is not known when the spring, noted for its excellent water (Varro, <i>Ling</i>. 5.71; Frontin., <i>loc. cit.</i>), was first associated with Iuturna. The pre-Augustan phases are normally ascribed to L. Aemilius Paullus (after the battle of Pydna in 168 B.C.) and to L. Caecilius Metellus in conjunction with his restoration of the Temple of *Castor in 117 B.C., when the basin was reduced to <i>c</i>. 7.5 x 7.5 m. In a third phase of development, the basin was further reduced to <i>c</i>. 5 x 5 m and lined with marble, a reworking normally associated with Tiberius’ rebuilding of the Temple of Castor in A.D. 6 (Steinby, <i>LTUR</i>). The Lacus Iuturnae is represented on the Severan Marble Plan (Carettoni <i>et al.</i>, <i>Pianta</i> pl. 21; Rodríguez, <i>Forma</i> pl. 13, frag. 18a).</p>