The novella’s striking imagery of the Congo emphasizes its liveliness and motion, which contrasts with the total stagnation of European cities like London and Brussels. For instance, when Marlow first arrives in the Congo in Part 1, he observes that:

The edge of a colossal jungle, so dark-green as to be almost black, fringed with white surf, ran straight, like a ruled line, far, far away along a blue sea whose glitter was blurred by a creeping mist. The sun was fierce, the land seemed to glisten and drip with steam.

Here, Conrad makes the Congo more vivid for his European readers by mixing visual imagery (like the jungle’s color and the glittering sea) with tactile imagery (like the sweltering sun and dripping land, which suggest sweat). The jungle is so dense that it’s “almost black,” and Marlow can’t even see into it. This represents his inability to understand it—indeed, in this passage Conrad contrasts the “Dark Continent,” which Marlow and his colleagues will struggle to conquer, with the white, shining sea, which they have crossed without a hitch. But Marlow can tell that the Congo is full of heat and activity, as it appears to “glisten and drip with steam.” In other words, it’s clear that he will find something in Africa, just not precisely what—after all, the rest of the book will bring the reader along on his journey. As it does, Marlow will again emphasize Africa’s impenetrability later in Part 1, by highlighting its seeming contradictions. For instance, he observes the contrasts between life and stagnation, or fullness and emptiness:

The great wall of vegetation, an exuberant and entangled mass of trunks, branches, leaves, boughs, festoons, motionless in the moonlight, was like a rioting invasion of soundless life, a rolling wave of plants, piled up, crested, ready to topple over the creek, to sweep every little man of us out of his little existence. And it moved not. A deadened burst of mighty splashes and snorts reached us from afar, as though an icthyosaurus had been taking a bath of glitter in the great river.

This is typical of Conrad’s descriptions of the forest. For instance, he suggests motion by presenting the forest as “exuberant and entangled,” then juxtaposing a series of nouns—“trunks, branches, leaves, boughs, festoons”—but then immediately shuts down this movement by clarifying that it’s all “motionless in the moonlight.” Similarly, he describes the plants as on the brink of motion, “ready to topple over the creek,” even though “it move[s] not.” The sounds of “mighty splashes and snorts” in the distance suggest that there’s some significant motion in the forest, but again, it’s not clear exactly what. These contrasts between movement and stagnation represent the consistent struggle in this book between nature—which tends toward life, creation, and motion—and civilization—which tends toward death, destruction, and stagnation. Later, Conrad more explicitly links imagery to human life in Part 2:

I made out, deep in the tangled gloom, naked breasts, arms, legs, glaring eyes—the bush was swarming with human limbs in movement, glistening of bronze colour. The twigs shook, swayed, and rustled, the arrows flew out of them, and then the shutter came to.

In this description, as a group of Congolese people attacks Marlow’s steamship, the rainforest is still a tangled mess. But the native people on the riverbanks blend in with the jungle itself, so that outsiders like Marlow cannot tell where plants end and humans begin. This continuity between native people and native plants represents the way that, in Conrad’s mind, rural non-Europeans live a more natural, “primitive” lifestyle. In contrast, Conrad sees the complex societies of urban Europe as obsessed with freezing things in place in order to own them—such as the ivory that they kill to obtain. While these ideas are no longer commonly accepted today, they are still the foundation for Conrad’s critique of European civilization in this novella, and he consistently incorporates them into his descriptions of the physical environment throughout it.