The submarine sunk in the opening flashback of Mission: Impossible – Dead Reckoning Part One is the Sevastopol.
The Sevastopol is a fictional Russian nuclear submarine carrying an advanced artificial intelligence system called the Entity. The system turns against the crew, causing the submarine’s destruction beneath the ice and beginning the chain of events that drives the film’s central conflict.
The vessel’s name is sometimes confused with the real Ukrainian port city of Sevastopol or with names of actual submarines. In the film, the Sevastopol is a fictional vessel and a key part of the Entity’s origin story. Its destruction is shown before the narrative shifts to the modern IMF mission involving Ethan Hunt.