Joseph Conrad's Heart of Darkness is based upon Conrad's own experience as a steamboat captain in the area once known as the Congo. Conrad commanded his ship for a short time when he became ill and had to return home, but he had seen enough of conditions in the Congo. Conrad bases the stations on actual locations within the Congo.
It is correct in thinking that with Conrad's work, there is more than meets the eye when studying his writing:
Conrad infused his work with psychological and moral implications. His characters face deep problems, ones with difficult or no answers. Their response to these questions often determines the course of their lives...much of his story lies beneath the surface narrative. The adventure is merely one level of the story...
The thread that joins all three stations together is the Congo River. The Congo River is symbolic of the journey a person takes to discover the truth about himself (or herself). The journey may offer items of beauty: flowers, exotic birds or other wildlife. However, Conrad's warns the reader of what lies beneath the beauty: mystery, seduction, or dangerous knowledge: corruption. In each person's soul, it can be found quietly waiting. In many people it may remain dormant forever; in other cases, it presents itself and a battle for control ensues. In some cases, even with the light, darkness is triumphant, and this is at the core of Conrad's novel.
The first stop is the Lower Station. This symbolizes warning and mental confusion. Marlow is bewildered by what he sees—a cemetery of old machinery lying about rusting for no reason. Men are blasting with dynamite; there is nothing in their way, so it is pointless, but they continue. He notices the natives who are treated like the enemy: they are beaten, shackled, starving and lacking hope. They have a look of death about them.
The experience of stopping at the Lower Station is disturbing to Marlow. Symbolically, it is a warning of an imbalance within society, an illness that has struck the men working for the Company: it is a moral disease.
Marlow travels to the Central Station. He finds his ship is underwater; repairs must be made. Symbolically, this, too, is a warning: an obstacle stops one from moving further into the darkness.
Here again, things don't make sense. The parts Marlow needs do not arrive—repeatedly. He meets a brickmaker, though there are no bricks in sight. There is a hidden urgency here about Kurtz, and a resentment for him. Again, the darkness is present, but it becomes more pronounced, harder to predict or pass through. Marlow is troubled.
Finally, Marlow reaches the Inner Station, where the darkness is the strongest. He has been attacked by natives firing on the ship. Kurtz's home is surrounded by heads on stakes. There are signs of human sacrifice.
Here morality is gone, sacrificed on an alter of greed and insanity. Here the darkness is at its most dangerous. Marlow is seemingly able to resist it, but Kurtz has succumbed, so much that his soul has gone insane and cannot be retrieved. Kurtz is brought out of the jungle by Marlow, but darkness's evil is too strong to save Kurtz.
Symbolically, each station represents the journey into the darkness of the heart and mind. That darkness can lure one in and destroy the soul. This could apply to any situation where one is tempted to turn away from the light, and allow darkness to enter. To continue leads to destruction; turning back to the light in time leads to rescue.