Sea Base 4, (PROSE: Warriors of the Deep) or Seabase Four, (PROSE: The Behemoths) was a sea base in the ocean on planet Earth. It was active in 2084 as part of Sea Base Command. It had several weapons systems including energy tracers.
In 2084, agents of a foreign power had already infiltrated Sea Base 4 with intent of sabotage. This scheme was undone when the base was attacked by the Sea Devils and the Silurians, who hoped to provoke mankind's two power blocks into a mutually destructive war. The humans were all massacred, before the Sea Devils and Silurians were killed by hexachromite gas released by the Fifth Doctor. As Turlough explained, "They're all dead, you know." (TV: Warriors of the Deep)
Behind the scenes
In the novelisation of Warriors of the Deep, it was mentioned there were several human survivors, including Bulic, scattered around the base.