Few people know that most of the damage from the great San Francisco Earthquake of April 18, 1906, resulted from the ensuing fires—the worst in US history. In 1908, the city's chief engineer conducted a study of 250 cities worldwide to develop plans for a guaranteed water supply. This paper describes the Auxiliary Water Supply System for fire protection, beginning with the original system which was approved by voters in 1908 Also included are an update of the current status of the system and details of the new SCADA system.