san francisco. they had no idea what they were facing. and it's our sound effect today. >> call for a fourth alarm for this. look, it appears that we have a plane down in the neighborhood. multiple structures on fire. and we have a fireball still coming out. >> stop that engine. we have no water in this hydrant. we need them to lay into us. >> copy that. >> we think we've got a broken water main down here so they need to lay in from the corner of san bruno avenue. >> this is north branch on 31, go. >> it does not appear that this is an aircraft down. it appears that this is some sort of natural gas explosion. >> quick thinking helped fire fighters overcome the destroyed water mains in the area. they improvised and stretched hose lines from another water grid. in all 400 first responders battled the fire. four people were killed, three still missing. 37 homes were destroyed. coming up, denzel washington