A picture of the small pool at the east end of the men's tepidarium. Please also note that the floor is supported by square brick columns. Warm air from the furnace outside would circulate among the columns and heat the floor that they supported—thus warming the room.
The Stabian Baths were the oldest and largest baths in Pompeii and were constructed in the second century B.C. They included a (un)dressing room (apodyterium), a medium temperature room (tepidarium), a hot room (caldarium), and a cold room (frigidarium). One usually proceeded through the bath in that order. Besides these usual rooms, they included a men's and a women's sections (thus baths), a large open exercise area (palestra), and a swimming pool.
The earthquake of A.D. 62 severely damaged these baths and some areas were not in use at the time of the eruption of Vesuvius.