Possible Clues:

• "At the ___ Core" (Edgar Rice Burroughs novel)
• "At the __ Core": Burroughs novel
• "At the ___ Core" (Burroughs novel)
• "Core" of an Edgar Rice Burroughs title
• Brownish pigments
• Cerium and samarium are rare ones
• "At the ___ Core"
• Multiple of these may exist in a multiverse
• They make things less shocking for the foxes
• ''At the ___ Core'' (1976 film)