A powerful explosion rocked Tacoma Saturday afternoon, sending four injured workers to the hospital, shooting a fireball hundreds of feet into the sky, shutting down highways and banning aircraft for a five-mile radius. The series of events leading to the explosion began as a tanker truck driver was loading propane into one of two stationary tanks at Atlas Castings & Technology. Two smaller explosions occurred, followed by a massive one that blew the truck apart, with the axel flying hundreds of feet in the air before