Pl.: piscinae
Also calida piscina

A large swimming pool heated by a hypocaust. In the centre of the pool, a metal container (samovar) connected to the hypocaust could be sunk into the bottom to ensure the continuous heating of the water. These pools are uncommon and mainly found in more luxurious bathhouses (both public and private). They were always confined to a separate room connected with the tepidarium or the caldarium.

L: The piscina calida in the Suburban Baths of Pompeii. Note the structure for the samovar at the centre (photo Konogan Beaufay).
R: The piscina calida in the villa of Massaciuccoli, in Tuscany (photo Allison Smith).