The baths fell out of use in the 6th century when the aqueduct was destroyed during a Gothic invasion of Rome.
Wikipedia has a great listing of the estimated amounts of stones used, many of which were imported:
Pozzolanna: 341,000 m³
Quick lime: 35,000 m³
Tuff: 341,000 m³
Basalt for foundations: 150,000 m³
Brick pieces for facing: 17.5 million
Large Bricks: 520,000
Marble columns in Central block: 252
Marble for columns and decorations: 6,300 m³
The floors were covered with mosaics - some black and white and others brightly colored from imported stones (yellow, green, purple, grey, pink).
There were also marble statues throughout the bath - such as these two. The first is the Farnese Hercules and the second is the Farnese Bull (portraying the binding of Dirce to a mad bull). One statue of Asclepius was over 4 meters tall.
1600 people could visit the bath at a time. As you can imagine, the baths must have been very noisy. There are written accounts of visitors complaining about the deafening level of noises.
Bathing and going to the bathroom were open and public - people did not feel the same modesty that we do today. Toilets were out in the open. Waste was carried out of the latrines through a channel of water that led to a sewer system.
People could exercise in the open courtyards (palaestra) - clothed or nude. The areas measured 1,076 x 1,315 ft (328X400m). Men typically lifted weights, wrestled, or played hand ball while women tended to swim in the large outdoor pools (called natatio) or play Trochus (a game where the player would roll a metal hoop with a stick).
Visitors could change their clothes in the Apodyterium (changing/dressing room). They kept their items in little cubbies or cubicles that had no locks. A capsarius, a slave attending the cubicles, could watch the belongings for a fee. The wealthy brought their own slaves to watch their stuff, carry their bath supplies, etc.
After working up a sweat, the visitor would then head to the tepidarium (warm water bath) before going to the caldarium (hot water bath) and then finally the frigidarium (cold water bath).
The caldarium measured 115 ft wide (35 meters) and topped with a dome. The waters were heated by an underground furnace in a system called a hypocaust. The hot steam would move through the floor and between the walls, escaping out hollowed-out spaces. It was stoked with wood by slaves. The heated pool was about 3 feet deep. A slave might pour cool water over the visitor in a dish called a patara to keep them from overheating.
The frigidarium measured 183 ft x 79 ft (55.7 meters x 24 meters).
In addition to scraping off dirt, the workers would pluck hair off the visitor's body since hairless bodies were en vogue. Sounds painful. They could also get massages.
There were other forms of entertainment available. Two libraries, one of Latin writings and one of Greek writings, were available as well as a theatre for poetry reading or plays. There were also shops, restaurants or cafes, an athletic track, gardens, salons, museums and music rooms.
Interestingly, people during this period thought going to the baths made them healthier, but quite the opposite was true. People who were ill shared the same bath water, so diseases such as tuberculosis, bowel issues, lice, typhus, and even malaria were spread here. In the second century, Emperor Hadrian imposed specific hours for bathers with illnesses (which probably really didn't matter since the diseases could live in the warm waters for long periods of time - ewww!)
PBS - Interactive Tour
A View on Cities
Wikipedia - Baths of Caracalla
Wikipedia - Caracalla
Every day life in Ancient Rome:
Ancient Mysteries - Incredible Monuments of Rome:
When Rome Ruled:
MrJennings - http://www.flickr.com/photos/mrjennings/120267581/