Earlier in the chapter the narrator has spoken of a "gloomy circle of some inferno", clearly referring to the topography of hell in Dante's poem. What is there, in the text, that justifies the association?
Joseph Conrad refers to the world in which his protagonist, Marlowe, has descended on his journey into the dense jungles of central Africa as hell, so miserable are the conditions he encounters. The lifeless, emaciated bodies, the disease, the horrible conditions under which the native people are forced to work and live, all conjure up images best associated with Dante's "circles of hell."
Allegories like that employed by Conrad are common literary devices. "Hell" is a particularly common metaphor for a situation so distasteful that no other word or description seems appropriate. Dante's "Divine Comedy" took the metaphor of "hell" to a new level, and its use implies a situation so horrific that a mere reference to "hell," as opposed to "Dante's nine circles of hell," will simply not suffice.