Action Five

In Bowie Base One protocol, Action Five was the last option in case of crisis: the complete destruction of the base. It would be performed by detonating the nuclear device at the heart of the Central Dome.

In the original timeline, on 21 November 2059, Captain Adelaide Brooke initiated Action Five after Bowie Base One was infiltrated by the Flood, with no options left. Due to communication delays, no one on Earth knew why he'd activated the nuclear device that day. It took her and her entire crew.

In a new timeline in which the Tenth Doctor intervened in events, Action One was briefly attempted, when Adelaide and the Doctor deduced that the crew would not already be infected, but the Flood got in, and took many in her crew. In the end, in order to keep the Flood from ever reaching the Earth, Adelaide did initiate Action Five, but the Doctor rescued her, Mia Bennett and Yuri Kerenski in the TARDIS, bringing them safely to Earth. However, Adelaide committed suicide in her home, in order to preserve the timeline. (TV: The Waters of Mars)