Mogilnoe, Natural monument lake in Kildin Island, Russia.

Lake Mogilnoe is a small meromictic lake on Kildin Island, 560 m long and 280 m wide, with an area of 96,000 m². Its water is layered by salinity, from nearly fresh at the surface (no more than 3‰) to over 30‰ at the bottom, and the depth of the halocline shifts from year to year. A 70 m wide, 5.5 m high ridge in the south separates it from the Kildin Strait.

The lake has been studied by researchers since the late 1800s, who documented its unusual water stratification pattern. This scientific work eventually led to its designation as a protected natural monument.

The Russian name Mogilnoye means "tomb lake". In 1916 a project was proposed to cut a canal linking the lake to the sea and build a shelter for ships there, but it found no support. Since 1985 the lake has held the status of a state nature monument.

The lake lies on Kildin Island between the settlement of Vostochny Kildin and Cape Mogilny, beside the Kildin Strait. The island is reached by boat from the Murmansk coast, and conditions on the strait change quickly, so plan for wind, fog and rain in any season. The water is layered by salinity and not suited to swimming.

The salt comes not through underground channels but by seepage through a 70 m wide, 5.5 m high barrier ridge in the southern part of the lake. Over a thousand years this seepage balanced fresh and marine water, so the lake holds four layers; only purple sulphur-producing bacteria live in the lowest, at 33 ‰ salinity.