from <http://people.clemson.edu/~elizab/rdlimages.htm>

Ruins of an Ancient Hypocaust

This isn't the hypocaust at the Stabian Baths, but it is a good example of what they look like. You can see the pillars which supported the floor, and the remains of the floor. The arch at the rear of the room was where the furnace was, and where the hot air from the furnace entered the space under the floor.