The Baths of Caracalla
Elaborate public baths constructed by the Emperor Caracalla around 216 CE, were a center of Roman social life and one of the great engineering triumphs of the 3rd Century. Sprawling over some 33 acres on Rome’s outskirts, the baths were a vast complex of business and entertainment establishments. At the center of everything were the baths themselves - a “frigidarium” (cold bath), several “tepidaria” (warm baths) and a “calidarium” (steam bath); most bathers passed through them in that order. Aqueducts fed thousands of gallons of mountain water into the system. Water for the tepidaria and calidarium was heated by the wood-burning furnaces connected to a network of steam pipes beneath the floors. The baths would remain in use until the 6th century when Goths destroyed aqueducts that supplied the baths with water.
(Source: last-of-the-romans, via et-haec)