The answer, I believe, is that the African landscape is one of the novella’s main characters. It is a character with an arc of its own, one that reflects Marlow’s evolving state of mind and eventually comes alive to play an active role in the story’s plot.
In the early scenes, the wilderness is only a vaguely menacing presence (“the river was there, fascinating—deadly—like a snake . . . almost featureless, as if still in the making, with an aspect of monotonous grimness”), but soon it begins to have an effect on—or to affect, depending on how you choose to read it—Marlow’s inner state:
. . . a general sense of vague and oppressive wonder grew upon me. It was like a weary pilgrimage amongst hints of nightmares.
Gradually, Marlow’s descriptions of the land become even more pointed:
. . . the silent wilderness surrounding this cleared speck on the earth struck me as something great and invincible, like evil or truth. . .
The landscape is silent with expectation. Marlow’s evolving reaction to it not only furthers the plot—he’s coming to terms with a force that will prove decisive in the story—but builds dramatic tension.
When Marlow hits upon the idea of getting rivets to fix the steamboat—which will enable him to make the journey up the river in search of the elusive Kurtz—he dances a noisy jig on the hollow metal deck. For the first time, the wilderness speaks back:
A frightful clatter came out of that hulk, and the virgin forest on the other side sent it back in a thundering roll . . .
The forest’s echo can be read as a warning. It’s also an important moment for a more practical reason: it lays the groundwork for the wilderness to speak, and even to take action, later in the story.
As the story develops, we close in on the climactic realization of the true character of the wilderness. The jungle seems to look at Marlow “with a vengeful aspect.” Unlike shackled nature back in Europe, the wilderness of Africa is malicious and dangerous, “monstrous and free.”
There is a hopeful moment on the upriver journey when Marlow catches a glimpse of something —the fog lifts and he has a clear view of the towering, still, matted jungle with the sun hanging like a ball above it—but then
. . . the white shutter came down again, smoothly, as if sliding in creased grooves.
It proves to be a fleeting vision, a teasing hint of light in an otherwise uninterrupted descent into the Heart of Darkness. For the reader, the tension is increased because we cling to the hope, however slim, that Marlow’s determinedly pessimistic outlook toward the intentions of the powerful force he is about to confront may prove to be mistaken.
But no. As the steamboat nears its destination Marlow begins to get a clearer sense of the true character of the wilderness. Just below Kurtz’s camp—right before the boat is attacked by flying arrows—he notices a string of sand bars, visible under the water as “a man’s backbone is seen running down the middle of his back under the skin.” It’s a deathly image, as if a giant has drowned or is waiting underwater to rise up and—and what, exactly? One is compelled to read on.
In the next few scenes the vision of the menacing living forest comes into focus. When Kurtz speaks as if he owns the entire African jungle Marlow expects to hear the wilderness “break into a prodigious peal of laughter that would shake the fixed stars in their places.” The forest is not only a character that speaks and ridicules; at the climactic moment of the story it also begins to act:
Instantly, in the emptiness of the landscape, a cry arose whose shrillness pierced the still air like a sharp arrow flying straight into the very heart of the land; and, as if by enchantment, streams of human beings . . . with spears in their hands, with bows, with shields, with wild glances and savage movements, were poured into the clearing by the dark-faced and pensive forest.
The wilderness thus plays a key role in the central conflict of the story; in the final encounters with Kurtz it is ever-present, an irresistible, crushing force.
Conrad’s vision of the wilderness is not mine; I tend to see nature as a redeeming rather than a corrupting force. My overall reaction to Heart of Darkness is probably colored by this fundamental clash of philosophies, but I must say that I found the story rather insistent in its continual return to the landscape as a brooding, menacing presence. It’s almost as if Conrad had discovered a new technique and was afraid to lose sight of it, the way a man might make repeated and needless trips out to the garage to wax and polish a new car.
Nevertheless, Heart of Darkness is notable for its clear and powerful use of setting: to build tension, to reflect the POV character’s evolving emotional state, and indeed to play a major role in the story as a character capable of action in its own right.
If you're interested in reading more on the importance of descriptive writing in novels, click here.