Kitezh, the city that sank into Lake Svetloyar
The legend of Kitezh says that when Batu's army came in 1238 the holy city sank into Lake Svetloyar (or became invisible), and that the pure in heart can still hear its bells; its written form, the 'Kitezh Chronicle', circulated among Old Believers from about the 1780s.
1780 CE · 56.82, 45.09 · status: legend
The account
Old Believers made pilgrimages to the lake on the eve of the feast of the Vladimir icon, praying at night and listening for bells. Writers such as Korolenko and Melnikov-Pechersky described the gatherings, and Rimsky-Korsakov's opera 'The Legend of the Invisible City of Kitezh' (1907) made it famous. The year given is the approximate earliest written attestation.
The sceptical view
Lake surveys found no sunken town; the legend is a religious vision of a hidden righteous Russia.