The mineral collectors in the photo are looking for hanksite, a rare evaporite mineral that ocurs at Searles Lake, near Trona, California.
One weekend a year, in October, Searles Valley Minerals opens the lakebed to collectors. Several days earlier, the company drills a row of holes down to an evaporite layer that contains hanksite. They set an explosive charge at the bottom of each hole to loosen things up.
As collectors watch, air from a large compressor is pumped down, forcing a brine/salt crystal mix to flow up through a large pipe. As the flood subsides, everyone jumps in to see what they can find.
The plant in the distant background extracts borax, sodium sulfate, sodium carbonate and sodium chloride from brines pumped from below the dry lake surface. In 1873, mineral extraction began with borax and raw trona scraped from the surface. Brine processing began in 1914 with recovery of borax and potassium chloride.
The Searles Lake evaporite beds were formed by desiccation of runoff from the eastern Sierra Nevada range and alternate with mud beds that formed during periods when the lake was full. Overflow spilled into Lake Manley, which filled Death Valley during the Pleistocene.
The Searles Lake Gem & Mineral Society, with support from Searles Valley Minerals, sponsors the fun and extraordinarily popular salt mineral field trips, usually in early October. Here's a link to their flyer: http://www1.iwvisp.com/tronagemclub/FLYER.htm