The Basilisk Revelation

Harry and Ron visit the Petrified Hermione in the hospital wing and find a page from an old library book clutched in her hand. The page describes a Basilisk, a giant serpent whose gaze is fatal, and notes that spiders flee from it. Hermione has also written the word 'pipes' at the bottom. Harry realizes that the monster in the Chamber is a Basilisk, which explains why he could hear its voice (as a Parselmouth) and why no one else could. The Basilisk has been moving through the school's plumbing system. The revelation also explains why the victims were Petrified rather than killed: they saw the Basilisk indirectly, through a reflection, a camera, or a ghost. The trio now knows what they are dealing with.