Lake Mogilnoe (Kildin Island, the Barents Sea) is a meromictic lake located in the high latitudes of the Arctic. It was formed by separation from the Barents Sea and retained an underground connection with it through a rocky barrier. The first researchers of the lake mentioned the upper fresh-water layer, the middle salty layer, lower anaerobic layer containing hydrogen sulfide, and noted the existence of a layer with rose water in the redoxcline. Observations carried out during the 20th century pointed to a gradual change in the vertical structure of the lake: mineralization increase of the surface layer and the rise of the hydrogen sulfide distribution boundary. In the past 10 years, there have been signals about acceleration of these changes. This is a big concern about an isolated population of Kildin cod (G. morhua kildinensis) listed in the Red Book of the Russian Federation, an endemic to this lake subspecies of Atlantic cod. In July, August and October 2018, three expeditions were organized to Lake Mogilnoe to study the current hydrological state of the lake, seasonal variability of hydrological characteristics, and outline possible causes of the registered changes and the threat to the Kildin cod population.