On her endeavor to construct the Theronian Bomb, Samus had to enter the Xenoresearch to acquire the Seeker Missile. In order to get into the containment area containing it, she had to deactivate the shields on all of the containment areas in the facility. This released all of the Phazon Metroids, which subsequently moved on to infest several other areas of own. The Metroids later move out of the area after the Elysia Leviathan's destruction, but the music which plays once they are released still plays, and they will occasionally phase in front of Samus in the rooms beneath the labs (but not in the actual labs themselves).
This area shows both advanced levels of decay and corruption. It is likely that the Pirates that were using this area contributed to both. The Phazon corruption was likely a result of the Pirates' experiments.