4. Ice cave on Olkhon Island on Lake Baikal in Siberia
Lake Baikal is the largest freshwater lake by volume in the world, containing roughly 20% of the world’s unfrozen surface fresh water. With 23,615.39 km3 (5,670 cu mi) of fresh water, it contains more water than the North American Great Lakes combined. With a maximum depth of 1,642 m (5,387 ft), Baikal is the world’s deepest lake. It is considered among the world’s clearest lakes and is considered the world’s oldest lake — at 25 million years. It is the seventh-largest lake in the world by surface area.