Yellowstone is a 132 square mile, 7733 feet high natural lake, the largest in North America, more than 7,000 feet high altitude lake, and one of the world's largest high altitude lakes. It is 20 miles long, 14 miles wide, 141 miles perimeter, an average depth of 140 feet, and an average annual water temperature of 5 degrees Celsius. The entire Yellowstone Lake contains 340,000 tons of lake water, and the annual outflow is 30,000 tons. The lake water is completely replaced every8-10 years. Observations since 1952 show that the height of the water surface of Yellowstone Lake fluctuates no more than 6 feet annually. Yellowstone Lake has a half-year freeze period from the end of December to the end of May and early June, and the ice layer will be several inches to two feet thick. Depending on the size, depth and local monsoon, Yellowstone Lake is sometimes like a rough inland sea. The Yellowstone Lake in late summer is layered due to different water temperatures. The surface water temperature of the lake is rarely more than 19 degrees, and the water temperature of the bottom layers is much lower, so it is not suitable for swimming. Humans can only survive under this water temperature for20-30 minutes.