Friar Pedro Benito Cambón, the mission’s co-founder, directed Chumash laborers to build this filtration building in 1792 as part of the mission’s timber, rock, and tile water system. Highly regarded for his knowledge of irrigation, agriculture, and building construction, he oversaw a system that by 1815 ended a seven-mile-long cobble and mortar aqueduct from the Ventura River and San Antonio Creek junction, furnishing the mission with water until floods destroyed the system in 1862. Water flowed into the barrel-vaulted structure’s main interior settling tank and then out through two underground tile pipelines to three locations. An adobe cistern behind the church still stands on the grounds of Holy Cross School, and a lavanderia tank for washing clothes was uncovered across from the church. The nearby fountain was nicknamed El Caballo because its mouth, now seriously eroded, was carved in the shape of an animal head, perhaps a horse. Only three such filtration buildings remain in the entire California Mission System, and Ventura’s is the oldest standing structure in the county. In the late 1800s it served as a jail “holding tank,” and now it remains a reminder of the Mission Period’s technical achievements.