"I turned to the wilderness really, not to Mr. Kurtz, who, I was ready to admit, was as good as buried. And for a moment it seemed to me as if I also was buried in a vast grave full of unspeakable secrets. I felt an intolerable weight oppressing my breast, the smell of the damp earth, the unseen presence of victorious corruption, the darkness of an impenetrable night."

This quote very closely reveals the inner struggles that marlow is feeling at this point in the book, and how he can only try to releave it with his comfort and unyielding friendship with nature.