The CS U1 is a vehicle in Just Cause 4: Sakaku.
It's the only nuclear submarine, or even submarine Sakaku has ever had.
The file from the Sakakan Ministry of Defense reveals:
This submarine was built in 1906, around the time the British Royal Navy built their HMS Dreadnought. As part of the modernization program the British sought to expand to their colonies, Sakaku ended up as the location of the first submarine the British built. During World War I, the ship was mainly on defense duty. During World War II, the Japanese captured the British garrison, but then-HMS U1 escaped capture and eluded Japanese patrols for over 4 years. Before the Cold War, the British handed the submarine over as a parting gift and was christened the now CS U1. During the Cold War, the submarine with its mainly Soviet crew received a major overhaul and performed reconnaissance on U.S. Navy ships in the Pacific. In the final days of the Soviet Union, the Sakakan crew revolted and bloodily killed the Soviet crew members. Today, it's the only platform where nuclear missiles can be launched.
It was still active as of September 22nd, 2050.