Quarantine Cave

From Wikitroid, the Metroid wiki

The Quarantine Cave is where you acquire the Spider Ball. When you first enter the cave, you will see a pile of large rocks. They aren't any rocks, they're Thardus, a rocky phazon-enhanced monster. Once you defeat Thardus, you get the Spider Ball and the only exit is to use the Spider Ball.In Space Pirate logs they keep Thardus there because they might have no use for him any more and since he's so big he can't get out.