Near Terme di Caracalla there is the biggest Mithraeum in Rome, where the main hall measures 23m by 10m. In the middle, there is a ditch connected to an underground drain. It could be the "fossa sanguinis", the place where the new followers laid down and were poured with the blood of a bull sacrificed on the upper floor. However, some researchers are skeptical about this as the spaces are not wide enough and difficult to reach to sacrifice a bull.