Baikal, the largest and deepest freshwater lake in the world, contains more than 1,200 rare species of plants and animals, including the world's only freshwater seals. Now it has become the centerpiece of the emerging Russian environmental movement, as nearby factories threaten its vitality. Here Matthiessen details its fate. A portion of the royalties to go to Baikal Watch to save the lake. 50 color photographs; 1 map.