The Great Salt Plains selenite

In 2007 a Boy Scout digging for crystals here uncovered vials of mustard gas.

Read that before you plan a trip, because it shapes the rules. The Great Salt Plains were used as a bombing and strafing range during the Second World War and remnants of that use are still on the plains. The vials the scout found were 7.5-inch containers holding agents including mustard gas, lewisite, chloropicrin and phosgene, part of wartime chemical agent identification sets that were buried as standard procedure for taking them out of service, with no disposal site ever recorded here. Crystal digging was suspended from April 2007 until April 2009 while the risk was assessed, and 171 vials and at least a dozen incendiary devices were removed.

Digging is allowed again, and the crystals are worth the trip. A designated area within the 11,000 acres of salt flat has gypsum concentrations high enough to grow selenite, a crystalline form of gypsum, and the crystals here carry an hourglass-shaped sand inclusion that is not known to occur in selenite anywhere else in the world. The season is fixed: 1 April through 15 October only, to protect whooping crane habitat.

The Oklahoma Historical Society describes the plains as a flat, bare, eleven-thousand-acre slab of land in Alfalfa County, named for the thin layer of salt covering the whole area. The salt was laid down by a shallow sea rising repeatedly over millions of years, and it is renewed constantly: saline groundwater flows a few feet below the surface, evaporates, and leaves salt behind. The same process grows the selenite.

The wider site is the Salt Plains National Wildlife Refuge, 32,080 acres established by executive order of President Herbert Hoover on 26 March 1930 and designated a national natural landmark in 1983. It holds about 312 bird species and 30 mammal species, is critical habitat for the endangered whooping crane, and is one of the most important shorebird habitats in the Western Hemisphere. Ralstin Island out in the lake is closed to the public and carries more than 30,000 birds in breeding season.

The salt drew people long before it drew collectors. Bison and other game came for it, and the Osage were firmly in control of the plains by 1760, trading their surplus to other tribes.