Outoor bathing obviously has a longer history than indoor bathing and private outdoor open space is known to have been used for bathing since ancient times. Stepwells were used for this purpose in Ancient India. Gymnasiums in Ancient Greece contained bathing pools. The Romans introduced the practice of bathing in hot water. Where possible, they used mineral springs with naturally hot water. Where this was not available, they used artificial heating systems, as at the Baths of Caracalla. This idea spread throughout the Roman Empire. It influenced the making of hot baths in the Islamic world and in northern Europe — including the baths in the Belgian town of Spa which has given its name to this type of facility. The garden spa in hotels, when including hydrotherapy, can be seen as the modern equivalent of the ancient outdoor bathing facility.