Look at Lake Tahoe between winter storms and it appears calm, serene, a grand sapphire set in the rough folds of the Sierra Nevada. But no body of water this size ever really rests. In December, the lake's surface begins to cool; the temperature will continue to drop until it's the same as that of the deep water below. All it takes then is the winds of a strong storm to turn the lake over. By February, the clearer water underneath has risen and researchers can sometimes see a white disk lowered to 130 feet below the surface.