|This article is written from the Real Life point of view|
It is used multiple times during the game, as a lead-in to the battles with the first Alpha, Gamma, Zeta and Omega Metroids. It is also heard in the cutscene where the first Metroid larva is seen draining a Hornoad; in the cutscene when the Metroid Detector reacts to Metroid larvae in Area 8; and in the post-credits ending when a Hornoad becomes infected by an X Parasite.
The theme consists of quiet droning ambience. In-game, its rather repetitive nature is mitigated by the fact that it is only heard for a few seconds before the respective Metroid appears, and (as is the case with the larvae and Omega Metroid) may even be obscured by sound effects to the point where it can only just barely be heard.
The theme can be heard here.