Lake Achit, or Achit Nuur in Mongolian, is the largest freshwater source spanning the periphery of Uvs Province in northwestern Mongolie. Within the mountainous Altai region, it serves as a natural border between Uvs and Bayan-Ölgii Provinces, Mongolia's westernmost territories.

Frozen from November through May, this lake has been shrinking gradually, its waters evaporating under the intense heat waves of recent years. Nestled within an enclave between two mountain ranges at 1,435 meters elevation, it feeds several major waterways: the Tsagaan, Khatuugiin, Bukhmurun, and Uliastai rivers.

The region has been designated a natural reserve, spanning the lake itself, the Khovd River to the southeast, and Dewel Island. Construction is strictly prohibited here; only seasonal activities such as nomadic herding are permitted. Limited, carefully regulated winter fishing is allowed.

This entire ecosystem exists to support the reproduction and reintroduction of local wildlife. The lake provides ideal nesting grounds for countless bird species alongside remarkable fish diversity and other mammals. It serves as a critical refuge for endangered species, including the Swan Goose, a threatened species that remains little-known, and Pallas's Sea-Eagle, a raptor found only in southern Russia, Mongolie, and northern China.

Another species successfully reintroduced in recent years is the Mongolian beaver, which has thrived and now inhabits Dewel Island.