At 4:20 p.m. on Sunday, September 29, 1957, a massive explosion occurred inside the perimeter of Chelyabinsk-40 in the southern Urals, a Sredmash installation so clandestine that it had never appeared on any civilian map. The forbidden area encompassed both the Mayak Production Association—a cluster of plutonium production reactors and radiochemical factories scraped from the wilderness by forced labor—and Ozersk, the comfortable closed city that housed the privileged technicians who staffed them.
