San Juan, Argentina, nestles in a fertile valley flanked by arid mountains. Croplands and vineyards (green) abut the metropolitan area on the San Juan River. The white teardrop (lower left) is an ancient lakebed called Barreal Blanco. It is one of the best places in the world for carrovelismo, or landsailing, thanks to the steady winds that sweep across this flat, unobstructed expanse of hard-packed sediment. March 17, 2003.