Lake Baikal (Russian Озеро Байкал (Ozero Baykal)), a lake in southern Siberia, Russia, between Irkutsk Oblast on the northwest and Buryatia on the southeast, near Irkutsk. Being 636 km long and 80 km wide, it is the largest freshwater lake in Asia (31,494 km²) and the deepest lake in the world (1743 m). Its age is estimated at 25-30 million years, making it one of the world's most ancient lakes in geological history.

The lake is completely surrounded by mountains and contains 22 small islands, the largest, Olkhon, being 72 kilometers long.. The lake is fed by some 300 inflowing rivers, the six main ones being Selenga, Chikoy, Khiloh, Uda, Barguzin and Upper Angara, and is drained through a single outlet, the Angara River.

The bottom of the lake is 1285 m below sea level and is the deepest continental rift on the earth. Its volume, — 23,000 km³ —, is approximately equal to the total volume of the 5 Great Lakes of North America, or to about 20% of the total fresh water on the earth.

Few lakes compete with Lake Baikal in biotic diversity. As many as 852 species and 233 varieties of algae and 1550 species and varieties of animals inhabitate the lake. The world-famous Baikal seal, the only mammal living on the lake, is found throughout the whole area of the lake.