|This article is written from the Real Life point of view|
X Invasion Detection (X侵入発覚) is an intense theme that plays in Metroid Fusion shortly after fighting Arachnus. Upon Samus's return to the Navigation Room, Adam will tell her that the X Parasites have invaded Sector 1, infecting the atmospheric stabilizers and changing the climate of Sector 1. Until Samus descends to the Main Elevator, this theme will play. It stops when the SA-X makes its first appearance.
The theme can be heard here: