A system of hot-air channels under the floors of the hot section of the bath, connected to the furnace by a vaulted tunnel. The hot air was evacuated over the roof through channels within the walls, providing additional heating and ensuring good circulation. In Karanis, the hypocaustum was a series of vaulted brick chambers and tunnels. More often, the floors of the heated rooms were made of large terracotta slabs supported in their corners on short brick piers. The solid basement in this bath took more time to warm up, but then conserved heat longer.