What is the core mystery surrounding the Metal Monster that challenges the main characters' beliefs?

The core mystery challenging the main characters' beliefs is the realization that the Metal Monster and its components are not merely inert metal, but are 'ALIVE AND THINKING'[1]. This phenomenon is described as 'outside all experience, beyond all knowledge or dream of science,' particularly because these metal entities move 'consciously, thoughtfully, deliberately'[1].

The characters struggle to comprehend 'Metal with a brain!'[1], as it defies their understanding of organic and inorganic matter and the very definition of intelligence and sentience[1].

Space: The Metal Monster By A. Merritt