Here is a great not-a-ufo photo from Reddit. It looks like something from the movie District 9 to me. True strange mothership stadiums of the planet earth.
Here is the likely source. Credit to the photographer, Alexander Lubyanchenko.
The [Krestovsky] stadium was opened in 2017 for the FIFA Confederations Cup. Initially, was planned to be completed by December 2008…As of May 2017, the stadium was 518% late and 548% over budget…At a cost of $1.1 billion at current exchange rates, it is considered one of the most expensive stadiums ever built.
(Russian: Газпром Арена) is a retractable roof stadium in the western portion of Krestovsky Islandin Saint Petersburg, Russia, which serves as home for FC Zenit Saint Petersburg
Here it is minus the fog: