A lost fighter squadron. A submarine adrift. A downed UFO. All part of the greater government conspiracy to keep secret the existence of extraterrestrial life on planet Earth, at least according to this two-part episode in season three of The X-Files. This is the episode that really developed the idea of the "black oil" - an oil-like substance that carries a virus which, when infecting a human (or humanoid, as it is later implied other species can be infected as well) leaves them as virtual slaves, like a particularly controlling parasite and its host. The virus in effect carries alien consciousness, though it is eventually discovered (in the first X-Files movie) that the virus mutates, and a corporeal alien will form inside the human host. Piper Maru also has the return of Alex Krycek, and his eventual infection by the oil and stranding at a secret military installation housing a UFC - to which the oil/virus/substance within him returns in a particularly chilling scene. A little bit of trivia: the Piper Maru (submarine) was named after Gillian Anderson's daughter.