Lake Brosno (Бросно), a 7.2-square-kilometre glacial lake in the Andreapol District of Tver Oblast in western Russia, is reputed to be the home of Russia's most thoroughly-catalogued freshwater cryptid. Local Tver tradition — going back at least to the 13th century and recorded in the chronicles of the Tver Principality — describes an enormous dark creature that emerges from the lake's depths to attack travelers along its shore and occasionally to capsize fishing boats. One of the most famous early accounts describes a Mongol Golden Horde detachment that, in approximately 1240 CE, halted its westward advance on Novgorod after a creature rising from Lake Brosno consumed several horses and men from the encampment.
The modern sighting catalog dates from the late 19th century. In 1854, a Russian Orthodox Imperial Navy officer named Prince Gagarin, surveying the lake for hydrographic purposes, reported observing a large dark shape 'at least 15 arshin' (approximately 10 metres) in length moving beneath the surface of the central deep basin. In 1996, geologist V. G. Potapov published a paper in the Moscow State University Geology Bulletin documenting the lake's 41-metre depth — unusual for the region — and the presence of a thermocline-layered hydrogen-sulfide lower column that would limit oxygenic habitation but could theoretically support unusual sulfide-tolerant species. In 2002, Moscow tabloid Komsomolskaya Pravda dispatched an expedition that recorded anomalous sonar returns at a depth of 37 metres consistent with a large organic mass.
Proposed identifications include the European catfish (Silurus glanis), which exceeds 3 metres and is present in the lake; an escaped sturgeon; a freshwater-surviving beluga seal; or — more speculatively — a surviving plesiosaur population in the lake's deepest sulfidic zone. The Tver Oblast government has declared Lake Brosno a regional protected area partly in consideration of its cultural-cryptid value. Local fishermen continue to report anomalous wakes and impressions of large animals in the lake's deepest section, and the creature (locally Brosnya) has become one of Russia's best-attested lake cryptids.