Friar Pedro Benito Cambón—the mission's co-founder was highly regarded for his knowledge of irrigation, agriculture and building construction—directed Chumash laborers to build this filtration building in 1792 as part of the mission's timber, rock and tile water system. By 1815 it was the terminus of 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. A lavanderia tank for washing clothes was uncovered across from the church. And this fountain, next to the filtration building, 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" but now it stands as a reminder of the great technical achievements of the Mission Period.