The Quarantine Cave is where Samus acquires the Spider Ball. When she first enters the cave, she sees a pile of large rocks. These rocks are actually Thardus, a Phazon-enhanced monster. Once Thardus is defeated, Samus gets the Spider Ball and the only way out is to use it. In Space Pirate logs it is revealed that they are keeping Thardus in Quarantine there because he became too powerful. After Thardus has been defeated, and Samus returns to this room, a Sheegoth will be slumbering where Thardus originally was.