A small but unusually deep lake in Tver Oblast — over forty metres, which is anomalous here. A tradition recorded in the chronicles has a dragon from the lake swallowing a detachment of Batu Khan's army on its way to Novgorod, after which the khan turned back. Fishermen describe a surfacing "head" and splashes that raise a wave on still water. Science offers something plainer: the lake carries hydrogen sulphide, and gas releases from the bottom periodically dome the surface.