Metroid Holding Areas, such as Metroid Quarantines A and B, are present in Metroid Prime for the purposes of containing Metroids. Sometimes the force fields detaining them must be disabled by Samus to access deeper parts of the area. Generally, the areas will also contain Xenome Containment Units to hold extra Metroids, Phazon, or mutated Saturnines.
"Metroid quarantine in effect. No unauthorized personnel in Holding Areas."