Lake Baikal.
Lake Baikal, Russia is one of the most beautiful and picturesque places not only in Siberia but also on the entire planet. Lying in a rift depression, located in the southern part of Eastern Siberia, this is the oldest lake in the world (being about 25-35 million years old).
Baikal is also the largest freshwater reservoir on Earth, so that 22% of all freshwater worldwide and 85% of Russia’s water supply are concentrated here. Its volume of water is 23 thousand km3 (approximately the amount of five Great Lakes combined in the United States).
In addition to the value of huge reserves of fresh water, (which, due to low mineralization: 100 g / l, it can be safely equated as distilled water) Baikal is also the deepest lake in the world. In 1996 it was included on the UNESCO World Heritage List.
https://www.lakebaikal.org/
The lake hosts an impressive unique flora and fauna. Scientists have identified about 2600 species of animals and plants and almost 70 per cent of them are endemic that means, they only can be found in this lake. One of the most representative inhabitants of the Baikal’s ecosystem is the Nerpa (Phoca sibirica), a unique freshwater seal which lives in the northern zone of Baikal and in the Ushkanye islands.
Another symbol of the Baikal’s fauna is the Omul, the most popular salmonid fish of the lake. Other representative specie of the lake is the Golomyanka also known as Baikal oil fish, an unusual beautiful fish, which is a translucent fish that live in depths between 200 and 500 meters, this kind of fish is famous for disintegrating and become a mass of oil and bones when they are exposed to sunlight. The region is inhabited also by foxes, eagles, deer, bears, and many others animals and plants.
https://7wonders.org/europe/russia/irkutsk/baikal-lake/
So, it is the oldest lake in the world. It is also the deepest lake in the world. It is also biodiverse.