A hypocaust was an ancient Roman underfloor heating system used to warm baths, houses, and other public buildings. The system worked by circulating hot air from a furnace through a series of hollow spaces beneath the floor and within the walls, heating the rooms above.
This innovation demonstrated Roman ingenuity and provided a level of comfort uncommon in the ancient world.
The hypocaust system at the Roman Baths in Bath, England, is a well-preserved example, showing how the Romans used this technology to heat their public bathing facilities even in colder climates.