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Lake Baikal is located in southern Russia, near the border of Mongolia. Its depth of 5,300 feet makes it the world's deepest lake. The second-deepest lake, Lake Tanganyika in east Africa, is 4,710 feet deep by comparison. Crater Lake, the deepest lake in the U.S., is 1,900 feet deep. Lake Baikal's 12,200-square-mile size also makes it Earth's largest. That size, by the way, makes it comparable in volume to the entire Amazon basin. For scale, it reportedly takes about 330 years for one water molecule to flow from inlet to inlet. So how did Lake Baikal get so massive? About 25 million years ago, Lake Baikal formed through fractures and shifting within Earth's crust. It wasn't Lake Baikal as we now know it, though. Experts believe it was a series of lakes, similar to the Great Lakes in the U.S. While scientists aren't positive how Lake Baikal went from many lakes to the behemoth it is today, they do have theories. It could've been sinking earth, erosion, earthquakes, increased water from melting glaciers, although it's likely a mix of these factors and more. Now, that unifying change took place in the Pliocene Epoch (about 5.3 to 2.5 million years ago), but this lake is hardly finished growing. It's expanding at a rate of 0.7 inches every year, the same speed at which Africa and South America are drifting apart. At this speed, some scientists believe Lake Baikal is actually an ocean in the making.