Lake Baikai Humanoids
The Lake Baikal Humanoids were aliens sighted in Russia in 1982. These beings were reported by the Russian Navy in Lake Baikal. Divers encountered humanoid beings wearing silver suits without any obvious breathing apparatus. Three people tried to follow them but ended up dying in the process. Due to the lake being really deep, large, and old, some think it is home to an underwater alien base.
Citations:
Steiger, Sherry Hansen, and Steiger, Brad. Real Aliens, Space Beings, and Creatures from Other Worlds. United Kingdom, Visible Ink Press, 2011.