The Baths of Agrippa appear to have featured the main three types of pools and rooms which were the staple of Roman baths: "frigidarium" (cold pool), "tepidarium" (mild/tepid pool), and "caldarium" (hot room and pool). There is research that suggests that these rooms could be visited in both orders, going from cold to hot, or hot to cold. The hottest of the rooms, the "caldarium", would have relied on heating from under the floors, created by fanning hot air from fires underneath the water basin, as well as heat from the sun, a feature which exploded after window glass became increasingly popular throughout Rome. It has also been suggested that the ringing of a bell ("tintinabulum") may have communicated to nearby Romans that the hot pools were open. However, this has also been criticized for various reasons of practicality, preferring the more reasonable explanation that bells were used to mark the imminent closing of the bathing complex.