Shasta was a near-twin of the Diablo shot fired a month earlier, and another of Livermore's steps toward a deployable hydrogen bomb. Detonated from a 152-metre tower at the Nevada Test Site on 18 August 1957, it tested a two-stage thermonuclear design at reduced scale: a boosted "Swan" primary — the compact fission trigger the laboratory was refining across Operation Plumbbob — was fired inside a mockup of the larger weapon it was meant to ignite. Shasta differed from Diablo only in small details of its casing and core geometry; the two shots, taken together, worked out the variables of a design too intricate to settle in a single firing. Shasta belonged to Operation Plumbbob, the longest and largest test series ever conducted on the American mainland — twenty-nine shots across the summer and autumn of 1957, and the most heavily contaminating of all the continental series. It was one of many developmental firings behind the compact primaries that would arm the thermonuclear warheads then entering the American arsenal.