The light at 1,400 m in Lake Baikal
During 1977 dives into Lake Baikal with Pisces deep-sea submersibles, Soviet crews reported a bright light at about 1,400 m that shone for a few seconds and went out.
1977 CE · 53.30, 108.00 · status: account
The account
The episode belongs to the same cluster of Baikal stories as the 1982 'swimmers' and is often listed in Russian USO collections. Unlike the swimmer story it is told as a crew observation from a scientific dive programme rather than a military secret. The exact dive and the people involved are rarely named in the retellings. The position given here is representative of the deep central basin.
The sceptical view
Skeptoid suggests a reflection or after-glow of the submersible's own lamps; bioluminescence is another ordinary candidate. Nothing about a few seconds of light points to a craft.