Ancient riverbeds eroded pockets, channels and depressions in the bedrock. After years of erosion and wind, these depressions are covered with sand. The ocassional precipitation in the Sahara filters throught the sand and pools in these ancient depressions. The water picks up minerals as it filters through the sand and these minerals are consentrated by further evaporation. Sometimes the sand is blown away to a level where water is exposed. Often these lakes are saltier than the sea.
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