They were second largest Roman public baths, or thermae, built in Rome between AD 212 and 216, during the reign of the Emperor Caracalla.
The baths remained in use until the 6th century when the hydraulic installations were destroyed. The bath was free and open to the public. The building was heated by a hypocaust, a system of burning coal and wood underneath the ground to heat water provided by a dedicated aqueduct. in the baths were found hot and cold pools, changing areas, restaurants and entertainment areas. It’s really quite spectacular!