St. Petersburg is the current name of the city that used to be Leningrad, on Earth in the former nation-state of Russia.
In 2268 Pavel Chekov claimed that Scotch whisky had been invented by a "little old lady" from Leningrad. (TOS: "The Trouble with Tribbles")
St. Petersburg was hit by the effects of the Whale Probe in 2286, losing all electrical power, and experiencing a cloud cover of 100% - blocking out the sun, and causing temperatures to plummet. The damage was reversed following the actions of James T. Kirk and the "HMS Bounty". (Star Trek IV: The Voyage Home)
- The city was renamed St. Petersburg in 1991, four years after the movie was filmed - causing a real life retcon.
- It is possible that the city's name switched back to Leningrad, when the USSR was reformed. And, also possible that Checkov was just really bad at making up history (since the city was not called Leningrad for the first time yet when scotch was "invented"). However, the 'Oblast,' or administrative region, surrounding the city of St. Petersburg is still called Leningrad.
- Fandom sources place Pavel A. Chekov's birthplace as St. Petersburg/Leningrad.