I can already see it in you through and through that you are already asking within yourselves: What does this center of gravity of the earth look like? What does it consist of? Is it a diamond lump, or is it pure gold, or iron, or even magnet? Or is it even a hollow space, filled with nothing but an eternally unquenchable fire, and serves perhaps even for the stay of the damned, and carries the respectable title: Hell, of which the fire-breathing mountains scattered here and there on earth are, in a way, chimneys? – The Earth, Chapter 1, Paragraph 12