This excerpt is from Heart of Darkness, by Joseph Conrad.
After this I got embraced, told to wear flannel, be sure to write often, and so on—and I left. In the street—I don’t know why—a queer feeling came to me that I was an imposter. Odd thing that I, who used to clear out for any part of the world at twenty-four hours’ notice, with less thought than most men give to the crossing of a street, had a moment—I won’t say of hesitation, but of startled pause, before this commonplace affair. 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.
Why does the protagonist feel that "I were about to set off for the centre of the earth"?