As the vast evergreen forests of Russia's Siberian taiga extend southward toward Mongolia, the ground rises and the terrain becomes more varied. The border between Siberian Russia and Mongolia is a natural divide here, with rugged hills and mountains forming series of wrinkles between the sprawling Russian forests to the north and rolling grasslands to the south. About midway along this border, in a gigantic stone bowl nearly four hundred miles (636 km) long and almost fifty miles (80 km) wide, lies almost one quarter of the all the fresh water on earth, Lake Baikal.
Geologists estimate that Lake Baikal formed somewhere 20-25 million years ago, during the Mesozoic. and, at more than 5,000 feet deep (1637m) at its most profound, with another four-mile-thick layer of sediment further down, it is the deepest lake in the world.
It's crystal blue waters are among the clearest as the water is oxygen rich and, containing roughly 20% of the world's unfrozen surface fresh water it is also the lake with the greatest volume in the world.
Lake Baikal is a UNESCO World Heritage site. It has a rare and diverse ecosystem to match. According to UNESCO, the lake is home to 1,340 animal species and 570 plants. Of these, 745 animals and 150 plants are found nowhere else on Earth.
Perhaps the cutest of these is the Baikal seal (Pusa sibirica), also known as the nerpa. This seal is the only pinniped that lives only in freshwater, according to the Seal Conservation Society. Adults are silver-grey, and pups are a fuzzy, fluffy white. Weighing in around 155 pounds (70 kilograms) max, Baikal seals are some of the smallest pinnipeds on the planet.
In the forests surrounding the lake there are an additional 10 threatened species along with the full complement of typical boreal species. The evolution of aquatic life that has taken place over this long period has resulted in an exceptionally unique and endemic fauna and flora. As the 'Galapagos of Russia', the lake is of exceptional value to evolutionary science.
In the winter Baikal freezes over, with ice so thick that the Trans-Siberian Railway was briefly run over its surface.
Despite its storied status, Lake Baikal is not immune to the threat of human activity. Baikal seals are hunted, which may be contributing to declining numbers of the species. Pollution also threatens the lake, particularly agricultural runoff and discharge from nearby industrial plants, according to the Seal Conservation Society.
In 2006, the Russian government announced plans to build the world's first International Uranium Enrichment Centre at an existing nuclear facility in Angarsk, 95 km (59 mi) from the lake's shores. However, critics and environmentalists argue it would be a disaster for the region and are urging the government to reconsider
The lake is also a repository of gas hydrates, which are essentially dissolved gases locked inside solid crystals of water. Lake Baikal hosts huge amounts of methane trapped in these structures in its depths, making it a popular place for research into how to extract these gas hydrates as an alternative source of energy.
Since 1992 Lake Baikal and the entire surrounding area have been designated as a national park, and Baikal is today a naturalist's paradise and an idyllic holiday destination.
Photo credits: Aask, Franziska Angele, Oleg Gekman, Ongm, Alexey Shevelev, Tiplyashina, Zastavkin.