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"I avoided a vast artificial hole somebody had been digging on the slope, the purpose of which I found it impossible to divine. It wasn't a quarry or a sandpit, anyhow. It was just a hole. It might have been connected with the philanthropic desire of giving the criminals something to do. I don't know. Then I nearly fell into a very narrow ravine, almost no more than a scar in the hillside." (1.38)
This random hole is another sign that people go crazy in the wilderness. (Maybe it's aliens?) But it isn't harmless eccentricity—when Marlow almost falls into another little ravine, we get the feeling that the madness is getting dangerous.
"He had come out for a moment, he said, 'to get a breath of fresh air. The expression sounded wonderfully odd, with its suggestion of sedentary desk-life." (1.43)
Wait, if you're actually living in the wilderness in a hut, why do you need to come outside for air? It's as though the accountant is trying to live the same kind of life he'd be living back in Brussels—which sounds pretty crazy to us.
"In the steady buzz of flies the homeward-bound agent was lying finished and insensible; the other, bent over his books, was making correct entries of perfectly correct transactions; and fifty feet below the doorstep I could see the still tree-tops of the grove of death." (1.48)
Marlow draws our attention to the madness of the situation by juxtaposing two very different images together—one of a man lying on his deathbed, and another of the accountant quietly going about his business as if nothing were wrong.