Lake Onega (/oʊˈnɛɡə/; also known as Onego; Russian: Оне́жское о́зеро, romanized: Onezhskoe ozero, IPA: [ɐˈnʲɛʂskəɪ ˈozʲɪrə]; Finnish: Ääninen, Äänisjärvi; Livvi: Oniegujärvi; Veps: Änine, Änižjärv) is a lake in northwestern Russia, on the territory of the Republic of Karelia, Leningrad Oblast and Vologda Oblast.

The eastern shores of the lake contain about 1,200 petroglyphs (rock engravings) dated to the 4th–2nd millennia BC, which have also been inscribed as a UNESCO World Heritage Site.

The lake is of glacial-tectonic origin and is a small remnant of a larger body of water which existed in this area during an Ice Age.

Sediments at that time – sandstone, sand, clay and limestone – form a 200-metre-thick (660 ft) layer covering the Baltic Shield which consists of granite, gneiss and greenstone.

Its level was first 7–9 m (23–30 ft) higher than at present, but it gradually lowered, thereby decreasing the sea area and forming several lakes in the Baltic region.

[4] They contain numerous elongated bays resulting in the lake's outline appearing similar to a giant crayfish.

In the northern part lies a large Zaonezhye Peninsula (Заонежье); south of it, is the Big Klimenetsky (Большой Клименецкий) island.

To the southwest of Greater Onega lies Petrozavodskoye Onego (Петрозаводское Онего) containing the large Petrozavodsk and small Yalguba and Pinguba bays.

The bottom has a very uneven profile, it is covered with silt, and contains numerous trenches of various size and shape in the northern part.

[9] The catchment area of 51,540 km2 (19,900 sq mi) drains into the lake via 58 rivers and more than 110 tributaries, including the Shuya, Suna, Vodla, Vytegra and Andoma.

The Onega Canal, which follows the southern banks of the lake, was built in 1818–1820 and 1845–1852 between Vytegra in the east and Svir in the west.

[1][8] Whereas the most famous is Kizhi, which contains historical wooden churches of the 18th century, the largest island is Big Klimenetsky with an area of 147 km2 (57 sq mi).

Common mammals include elks, brown bears, wolves, red foxes, European hares, blue hares, red squirrels, lynxes, pine martens, European badgers, as well as American muskrats and minks which were introduced to the area in the early 20th century.

[9][11][12] Lake Onega features a large variety of fish and water invertebrates, including relicts of the glacial period such as lamprey.

[7][8][13] The lake area used to be clean but pollution is gradually increasing, especially in the northwestern and northern parts which contain the industrial facilities of Petrozavodsk, Kondopoga and Medvezhyegorsk.

[14] The lake basin is a major source of granite, marble and black schist in Russia which have been exploited in the area since the early 18th century.

[14] Lake shores contain two ports (Petrozavodsk and Medvezhyegorsk), 5 wharves (Kondopoga, Povenets, Shala, Vytegra and Ascension (Вознесенье)) and 41 piers.

About 17 species are being fished commercially, mostly European cisco, smelt, whitefishes, roaches, burbot, pike-perch, perch, ruffe, carp bream, lake salmon, pike and somewhat less ide, graylings, common dace, common bleak and crucian carp.

[22] The largest city on the lake is Petrozavodsk – the capital of Republic of Karelia (about 270,000 citizens) – founded in 1703 by Peter I to exploit the natural ore deposits.

There are two carillons in the city, with 23 and 18 bells, also there is an indoor ice sports arena accommodating 1,850 spectators and a Palace of Arts with an organ.

During the World War II this area was occupied by the Finnish forces and was a place of busy military activities.

[29] In 2021, the petroglyphs were inscribed on the UNESCO World Heritage List for its significant artistic qualities that testify the creativity of the Stone Age.