Shot Whitney was the 22nd full-scale nuclear test of Operation Plumbbob, detonated on 23 September 1957 at 5:30 a.m. PDT from a 500-foot tower in Area 2 of the Nevada Test Site. It was a UCRL development test of a boosted Swan primary fired in a W-27 thermonuclear system mockup, producing 19 kilotons against a predicted yield of 15 kt. This Lookout Mountain Laboratory photograph — AEC-57-5862 — captures the cloud at a mature stage in the pre-dawn darkness of Yucca Flat: a massive, fully-developed mushroom cap rising above a dense, churning stem, the complex internal structure of the cloud visible in the contrast between its luminous outer edges and the dark material folded within. At ground level the base surge spreads across the desert floor. By the time of Whitney, Operation Plumbbob had been running for nearly four months — Whitney was among the final shots of the series, which concluded two weeks later with Shot Morgan on 7 October.