A 30-in. diameter steel underground natural gas transmission line owned and operated by the Pacific Gas and Electric Company (PG&E) suddenly ruptured in San Bruno, California on September 9, 2010. The subsequent explosion and fire resulted in the loss of eight lives, injuries to 67 individuals, and the total destruction of 38 homes. Additionally, 70 homes sustained damage, and 18 homes adjacent to the destroyed properties were left uninhabitable. A thorough investigation ensued to determine the actual cause of this tragic event. The National Transportation Safety Board's (NTSB) investigation concluded that the rupture of the pipeline was a result of a fracture that originated in the partially welded longitudinal seam of one of six short pipe sections, which are known in the industry as "pups." The fabrication of five of the pups in 1956 would not have met generally accepted industry quality control and welding standards then in effect, indicating that those standards were either overlooked or ignored. The San Bruno incident ranks as the most significant natural gas pipeline incident in terms of loss of life and property in recent years. This paper provides an overview of this tragic event and discussion of the investigation process.