Marlow uses a simile comparing the Congo River to a snake in order to explain why he decided to go there. He connects this comparison to his childhood interest in maps and exploration:
There was in it one river especially, a mighty big river, that you could see on the map, resembling an immense snake uncoiled, with its head in the sea, its body at rest curving afar over a vast country, and its tail lost in the depths of the land. And as I looked at the map of it in a shop-window, it fascinated me as a snake would a bird—a silly little bird.
In literal terms, this simile refers to the visual similarity between the shape of a snake and the shape of the Congo River, which arcs across Central Africa in a vast semi circle. However, snakes also carry other important symbolism—most notably, as in the biblical story of the Garden of Eden, snakes can represent evil, temptation, and the dangers of curiosity. In this sense, by describing the Congo River as a snake, Marlow suggests that his decision to go there was a foolish error: he gave into temptation, let his curiosity lead him astray, and paid the price (which he will explain later in the book, as he describes the perils that he faced on his journey).
Of course, birds often eat snakes, so by comparing himself to a bird who is fascinated by the snakelike Congo River, Marlow also hints at his and other Europeans’ real motives for traveling to Africa: they want to plunder the Congo’s resources and enrich themselves. Yet, by calling himself “a silly little bird” (rather than the kind of mighty bird of prey that could devour a snake), he foreshadows how the Congo stuns and overwhelms him. He never succeeds in plundering it—instead, it nearly kills him through a nasty tropical disease at the end of the book.
After describing his initial meeting with the dishonest Brickmaker, who repeatedly praises Kurtz but also clearly hates him, Marlow uses a vivid simile to sum up his own moral principles:
There is a taint of death, a flavour of mortality in lies—which is exactly what I hate and detest in the world—what I want to forget. It makes me miserable and sick, like biting something rotten would do.
Marlow’s point is straightforward: telling a lie is “like biting something rotten” because it provokes feelings of disgust and shame. Like a lie, rotten food lacks integrity—it may appear intact on the outside, but it’s broken and worthless on the inside. Thus, Marlow uses this simile to define his own firm values, much like Kurtz does in the pamphlet he writes about “the Suppression of Savage Customs.”
Yet, much like Kurtz, Marlow ends up abandoning his principles. As a result, this simile serves a far more important role in the novella’s plot as a whole. Just after this line, Marlow tells a lie: he convinces the Brickmaker that he’s influential in Europe because he thinks that it will “be of help to that Kurtz.” He doesn’t even fully understand why he’s doing it. Thus, Marlow’s simile also suggests to the reader that Marlow’s time in the Congo has started to morally rot him by turning him into a liar.
Moreover, after describing his lie to the Brickmaker, Marlow tells his audience (the four other sailors) that it feels impossible to accurately explain his experiences, feelings, and thoughts through his story. In other words, he asks whether storytelling (or literature) can ever really capture the truth at all, or whether all stories are really just made of a series of lies, whether greater or lesser, intentional or unintentional. Thus, he calls into question whether the story he is telling is really true, or just another one of the rotten lies that he learned to tell in the Congo.
At the Inner Station, the General Manager is delighted to see that Kurtz—the Company’s best ivory trader and his main rival—is dying. But instead of revealing his true feelings, he feigns grief. This disgusts Marlow, who uses a simile to compare his disappointment about meeting Kurtz (and the General Manager’s utter corruption) to being buried alive:
Mr. Kurtz [...] was as good as buried. For a moment it seemed to me as if I also were 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…
The “unspeakable secrets” Marlow describes are all of the ways in which the reality of Kurtz and the Company’s work differs from the stories that Marlow heard on his journey upriver. Kurtz is not a fit, principled missionary trying to spreadWestern civilization’s glorious values through the ivory trade—instead, he’s a uniquely ruthless businessman who’s willing to do anything (including giving up all of his principles) to turn a profit. The General Manager is selfish, dishonest, and cruel—just not as much as Kurtz, which is why he isn’t as successful of an ivory trader.
Facing the glaring contradictions between myth and reality, Marlow realizes that his entire expedition to the Congo has been based on a series of lies. There is no benevolent colonial government and no civilizing mission. There is just plunder, theft, and slavery, and now Marlow has become a part of it. This is why he feels like he’s being buried alive: the weight of being deceived, and accidentally deceiving himself in the process, feels too great to bear. In this sense, perhaps he’s just like Kurtz—who also might have really believed in his principles, once upon a time. Indeed, Marlow compares his situation to a grave because he notes that Kurtz is about to die and get buried. By comparing Kurtz’s upcoming burial to his own, Marlow highlights how the two men serve as character foils for each other and foreshadows the fact that he is about to contract the same severe illness as Kurtz (and nearly die from it).