A Caldarium (also called a Calidarium, Cella Caldaria or Cella Coctilium) was a room with a hot plunge bath, used in a Roman bath complex. This was a very hot and steamy room heated by a hypocaust, an underfloor heating system. This was the hottest room in the regular sequence of bathing rooms; after the caldarium, bathers would progress back through the tepidarium to the frigidarium. In the caldarium there'd be a bath (alveus, piscina calida or solium) of hot water sunk into the floor and ther… (More on Caldarium)