Since Marlow is narrating the story himself, much of the narration is styled to sound like a running monologue; he stops, goes back, remembers disconnected information and repeats himself. One method the author uses to create a threatening atmosphere is the repeating of phrases and themes: the "heart of darkness" itself is one, and Marlow seems to become enamored with tangents of what he felt rather than what happened. Long paragraphs are spoken without pause for breath, creating a solid mass of text that feels difficult, and even imprisoning.
Trees, trees, millions of trees, massive, immense, running up high; and at their foot, hugging the bank against the stream, crept the little begrimed steamboat, like a sluggish beetle crawling on the floor of a lofty portico. It made you feel very small, very lost...
(Conrad, Heart of Darkness, gutenberg.org)
Here, the repetition of "trees" and the descriptive words make the jungle feel claustrophobic and oppressive; the humans on the river are like "beetles" since they are so small in comparison with the enormous landscape. Their movement is in spite of the jungle, not in harmony with it, and at any moment those "massive, immense" structures -- trees or vines, or any other jungle feature -- might become overtly hostile.