Lake Baikal is the oldest lake in the world, estimated to be about 25 million years old.
It formed in an ancient continental rift in southern Siberia, where the Earth's crust is slowly pulling apart. This tectonic origin also helps explain Baikal's great depth and unusually long, narrow shape. Geological evidence indicates that the basin has remained a lake for millions of years, unlike many younger lakes created by glaciers.
Baikal is also the world's deepest lake and contains roughly one-fifth of the planet's unfrozen surface fresh water. Its age is sometimes confused with the age of its modern shoreline or individual sediments; the commonly cited figure refers to the long-lived lake basin and its continuous history.
The lake supports exceptional biodiversity, including the endemic Baikal seal, the only seal species found exclusively in fresh water.