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"The offing was barred by a black bank of clouds, and the tranquil waterway leading to the uttermost ends of the earth flowed somber under an overcast sky—seemed to lead into the heart of an immense darkness." (3.87)
That sound you heard was the anvil of symbolism. Conrad closes the novel with a scene of darkness, suggesting heavy-handedly that evil exists and no one cares. Awesome.
"I looked around, and I don't know why, but I assure you that never, never before, did this land, this river, this jungle, the very arch of this blazing sky, appear to me so hopeless and so dark, so impenetrable to human thought, so pitiless to human weakness." (3.3)
Nature seems to Marlow completely "hopeless" and "dark," completely inaccessible to the human mind, incomprehensible and merciless to human weakness.
"The woods were unmoved, like a mask—heavy, like the closed door of a prison - they looked with their air of hidden knowledge, of patient expectation, of unapproachable silence." (3.4)
Look, we get that Marlow is a little freaked about by all this nature, but we're starting to suspect that he's taking it too seriously. They're just trees. Right? Right??