The baths stood on a platform 6.1m (20ft) high and over one fifth a mile long and could accommodate 1,600 bathers. Underneath the platform were the vaulted store-chambers, corridors, furnaces, hypocausts and hot air ducts for heating the buildings. A colonnade on the entrance side screened two stories forming shops on the ground level and 'slipper' baths on the platform level. The main entrance led to a park-like enclosure. The baths also included halls for performances and lectures, and a stadium. On the opposite side of the platform was the vaulted reservoir of water supplied by the Marcian aqueduct.
"The Baths of Caracalla, the second largest baths complex in ancient Rome, were built between by the emperor Marcus Aurelius Antoninus, better known by his nickname Caracalla. By the 3C AD the Romans had built many baths, in Rome and elsewhere, and had acquired great skill in designing functional, fully integrated complexes. The water supply and drainage system, in particular, required careful planning to ensure an adequate flow to and from the numerous hot and cold basins: it has been calculated that the baths used 15-20,000 cubic meters of water per day. The baths were fed by a branch of the Aqua Marcia aqueduct, which brought pure water to Rome from springs in the hills near Subiaco, over 90 km away. The water flowed into a huge cistern, divided into 18 separate chambers for easy maintenance and with a total capacity of 10,000 cu. m. From here it went by gravity flow through pipes underneath the gardens to the main building." Source: www.inforoma.it/terme.htm