p. 10, What are some different things we might conceive to exist at the "center of the earth?" In what sense does this sentence turn out to be appropriate?

I think Marlow is apprehensive about this trip. It is unlike any other he had ever taken,

The best way I can explain it to you is by saying that, for a second or two, I felt as though, instead of going to the centre of a continent, I were about to set off for the centre of the earth."

The centre of the earth brings to mind a heat oppressed sort of Hell, a Dante-like vision of devils torturing the meek and sick. This isn't unlike the heat and white devil's he will encounter in the Congo.