People have been gathering crystalline salt since prehistoric times, both from the seacoasts and from inland salt deposits. The rock-salt deposits, some of which are hundreds of millions of years old, are masses of sodium chloride that crystallized when ancient seas were isolated by rising land masses and evaporated, and their beds then covered over by later geological processes. Until the 19th century, salt was produced mainly for the preservation and flavoring of foods. Nowadays large amounts are used in industrial manufacturing of all kinds, as well as in the de-icing of winter roads, and salt production itself has been industrialized. Most rock salt is now mined by solution, or pumping water into the deposits to dissolve the salt, then evaporating the brine down in vacuum chambers to form solid crystals. While some sea salt is still produced by gradual solar evaporation from open-air salt pans in sufficiently warm, dry regions, much is now produced by more rapid vacuum evaporation.