The giant ice rings of Lake Baikal
Dark rings 5 to 7 km across appear in Baikal's spring ice; after astronauts on the ISS photographed two in April 2009 they drew worldwide attention and some talk of craft beneath the ice.
2009 CE · 53.10, 107.80 · status: explained
The account
Russian researchers had seen them in satellite images in the early 2000s, and a search of archive imagery found rings back to 1969. Methane rising from the lake bed was a popular early explanation. Field work in 2016 and 2017 by Alexei Kouraev (University of Toulouse) and colleagues, published in Limnology and Oceanography in 2019, measured warm lens-shaped eddies turning clockwise under the ice: at the rim the stronger, warmer current thins the ice, while the slow centre stays thick. Rings were also found on Lake Hovsgol and Lake Teletskoye.
The sceptical view
The eddy mechanism is measured, not guessed; some details, such as why the eddies form where they do, are still studied.