A hypocaust is a system of central heating in a building that produces and circulates hot air below the floor of a room, and may also warm the walls with a series of pipes through which the hot air passes. This air can warm the upper floors as well. Hypocausts were used for heating hot baths and other public buildings in Ancient Rome. They were also used in private homes. It was a must for the villas of the wealthier merchant class throughout the Roman Empire. The ruins of Roman hypocausts have been found throughout Europe (for example in Italy, England, Spain, France, Switzerland, and Germany) and in Africa as well.

Source: Wikipedia