The natatio is circular, and its roof a dome or cone. It may have been painted blue above, but its present hue is nearly black. Of the scholae, or recesses, two are here shown, coloured blue with red alcoves. The walls were yellow, and in the centre, between the two recesses, a brazen spout threw the water into the piscina or cistern, across the ambulatory, which, like the vase, was covered with white marble.

Below the dome is a frieze, painted red and ornamented with what are now only the vestiges of bassi relievi in stucco, representing horse, chariot, and foot races. It is the only tolerably perfect natatio at present known to exist.