Place yourself in the heart of the Great Plains, say, somewhere in the Texas Panhandle. A long, lonely stretch of interstate extends before you. Prairie grass and planted wheat cover the landscape out to the horizon, flat as a table in all directions. But it's not truly flat. Even on these plains there are low spots, the ground sloping almost imperceptibly toward slight, bowl-shaped depressions where the infrequent rains of this semi-arid environment collect. These are playas: wetlands that come and go, providing an oasis for life in an otherwise desiccated place.
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