The Brosnensky also known as the Brosno dragon is a dragon-like cryptid, according to local beliefs, supposedly living in the lake Brosno in the Tver region of Russia. About the monster is known from the words of "eyewitnesses" local residents, most of them tell about the lizard in words, but some, allegedly, even "had the luck" to make his pictures.
According to descriptions of "eyewitnesses", this creature is scaly, like a reptile, at least five meters in size.
The first mentions of the "Brosny Monster" date back to the time of the Tatar-Mongol invasion. According to legend, Batyi's army stopped near the shore of the lake. At night the warriors heard a great splash of water, and then they saw a huge monster come ashore and attack the people. In terror, the army scattered. Khan considered it a bad sign and ordered the army to retreat.
According to local accounts, during World War II the monster swallowed one German pilot, who catapulted from a shot-down plane into the waters of Lake Brosno. Some claim to have seen the monster's scaly head above the lake surface even today.
The origin of the lake lizard legends was most likely due to the unusually deep depth of the lake, and the processes of decay at depth leading to the formation of hydrogen sulfide "bubbles." These are hydrogen sulfide hydrates, constantly accumulating at the bottom a stock of poisonous gas, due to the high pressure of the water column being in a bound state.
To "boil" the hydrate cushion, a slight external impact (a pop from an explosion, dropping an anchor, etc.) is enough. The consequences of such a trivial anchor drop at the wrong time in the wrong place can be catastrophic.
If there had been any eyewitnesses on the nearby hills, they could have seen such a disappearance in the abyss as an "open mouth" (with or without spewing fire) which had swallowed its victims. According to the descriptions of eyewitnesses the monster strongly resembles a prehistoric plesiosaurus.