Archaeologists have discovered the remains of a 2500-year-old advanced civilization at the bottom of Lake Issyk Kul in the Kyrgyz mountains in Russia.
According to a report in RIA Novosti, the team consisted of Kyrgyz historians, led by Vladimir Ploskikh, vice president of the Kyrgyz Academy of Sciences, and other Russian colleagues, like historian Svetlana Lukashova.
The expedition resulted in sensational finds, including the discovery of major settlements, presently buried underwater.
More here. There's not a lot of explanation as to why the lake levels rose except to attribute it to various events like earthquakes.