This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Most of the world's large lakes are geologically young and comparatively shallow, carved by glaciers or dammed by rivers within the last few tens of thousands of years, and most are also doomed on a long enough timescale, since sediment washing in from surrounding rivers slowly fills a lake basin in until it becomes a marsh and then dry land. Lake Baikal, in southern Siberia, breaks both patterns at once. It sits inside an active continental rift, a place where the Earth's crust is being pulled apart rather than compressed, and the same rifting that opened the basin roughly twenty to twenty five million years ago is still slowly widening it today, deepening the lake faster than sediment can fill it in. The result is a lake old enough that scientists study its sediment layers, hundreds of meters of them accumulated undisturbed over millions of years, as one of the most complete continuous climate records anywhere on the continent, and deep enough, at a maximum of about 1,700 meters, to hold roughly a fifth of all the unfrozen fresh water on the surface of the Earth in one basin.
That combination of age and isolation has made the lake into something closer to an inland ocean than an ordinary lake, biologically speaking. A body of fresh water that has remained stable and separated from other water systems for tens of millions of years gives evolution enough time and enough isolation to produce endemism on a scale freshwater lakes almost never reach, comparable in kind, if not in scale, to island archipelagos like the Galapagos. More than half of Lake Baikal's recorded species, by most estimates, exist nowhere else, spanning everything from sponges and flatworms to specialized deep-water fish adapted to the near-freezing, high-pressure conditions of the lake's lower depths, conditions closer to those in parts of the open ocean than to those in any other lake on the continent.