By the close of the Civil War twenty-five-year-old Timothy O'Sullivan had had seven years' experience in wet plate photography, five of them working from a van on or near the battlefield. His technical proficiency under adverse conditions and his strong constitution recommended him as a photographer for the Geological Exploration of the Fortieth Parallel, the first of several exploratory surveys of the American West. Clarence King, an enterprising geologist from Yale, had convinced the government to implement a study of the geological structure and natural resources of the region west of the Great Plains and east of California, the so-called Great American Desert. From 1867 to 1872, King and his corps of young scientists mapped and described a band one hundred miles wide by three hundred miles long lying roughly along the route of the railroad that would link the east and west coasts in 1869. The Humboldt Sink was perhaps the most desolate landscape studied by the expedition. Known locally as the "worst place between Missouri and Hell," the mountain-rimmed salt marsh was blisteringly hot, mosquito infested, and rank with sulfur and rotting vegetation. King went temporarily blind there, and fever prostrated most of the men. But O'Sullivan continued to work. In this photograph his portable developing box is visible in the left foreground; in the middle ground his van (a converted ambulance), dwarfed by the vastness, establishes scale and articulates the emptiness of the space. The print is inscribed on the verso "Valley 5,000 ft. above sea, East of East Humboldt Range. Group of Salt Geyser Springs/Salt Marsh Valley/97 percent salt 3 borax."