Which type of bath is known for its below-floor heating system in the Baths of Diocletian?

The hypocaust system is a significant architectural innovation used in Roman baths, including the Baths of Diocletian. This below-floor heating system allowed warm air from a furnace to circulate beneath the floors and through the walls, effectively heating both the rooms and the water in the baths. The hypocaust is particularly important because it facilitated the use of baths as social and recreational spaces, allowing for a comfortable environment regardless of external weather conditions.

The other types of baths mentioned—frigidarium, caldarium, and tepidarium—refer to specific bathing areas within the baths themselves. The frigidarium is the cold bath, the caldarium is the hot bath, and the tepidarium is the warm bath. While each of these areas plays a role in the overall bath experience, they do not refer to the heating system itself, which is uniquely defined by the hypocaust. Thus, the hypocaust stands out as a hallmark of Roman engineering and is fundamental to understanding the advancements in bathhouse design during that period.