Conrad's title, Heart of Darkness, refers most obviously to the interior of what was once known as the Congo in Africa—that had for many years, been a non-navigable source of mystery. The Congo was dangerous and foreign to those who ventured within—primarily to become rich from the area's natural resources: ivory, and later, rubber.
It is in the maniacal quest for power and wealth, and the abuse and murder of the Congo's natives, that Conrad brings to life through the observations of Marlow, the story's narrator. "Darkness" is the focus of the mystery, the inhuman behavior of the whites on the region's inhabitants, and finally, the loss of humanity Marlow continuously witnesses. This culminates in the realization of the madness that has beset Kurtz, who Marlow has traveled inland to bring home.
The mood or atmosphere is created with descriptions of the land, the behavior of those Marlow meets, and essentially in the "horror" (as Kurtz identifies it) of the degradation of "civilized" man and the destruction of the innocents in the Congo.
Marlow describes the wanton destruction of the land—the senseless destruction of a cliffside with dynamite—which serves no purpose, and the pollution of the landscape with litter:
I came upon a boiler...then...an undersized railway-truck lying there on its back with its wheels in the air...The thing looked as dead as the carcass of some animal.
The "Company" has shown a total disregard for this new territory—the explosions on the cliffside are without purpose: there is no road being cleared. Here are actions without intent or logic...perhaps a foreshadowing of the madness Marlow has yet to witness.
At the first station, Marlow sees the subjugation of the natives—enslaved: chained together, forced to work, starved, and killed—robbed of their humanity:
…these men...were called criminals...All their meager breasts panted together, the violently dilated nostrils quivered, the eyes stared stonily up-hill. They passed me within six inches, without a glance, with that complete, deathlike indifference of unhappy savages.
The whites have not only forced themselves upon the land, but also have victimized its occupants. This, too, contributes to the mood of "darkness" and brings to mind the theme of "horror," forms the reader's response to the story.
One of the most poignant scenes in Marlow's tale that drives the increasingly dark mood is what the steamboat captain witnesses as he arrives at the last station—where we discover that Kurtz is something of a god to the inhabitants of this innermost part of the jungle. Marlow's description is hard to forget, but drives home to the reader, the true horror in Kurtz's transformation:
Now I had suddenly a nearer view, and its first result was to make me throw my head back as if before a blow...I went carefully from post to post with my glass…These round knobs were not ornamental...they were expressive and puzzling, striking and disturbing—food for thought and also for vultures…They would have been even more impressive, those heads on the stakes, if their faces had not been turned to the house. Only one, the first I had made out, was facing my way.
The heads of the dead on the stakes surrounding the building bring to the reader the depths of the horror hidden in the center of the Congo region where Kurtz has lost his mind, driven by his quest for more ivory. Conrad's imagery dramatically creates a frightening atmosphere of darkness and horror.