How could Marlow's journey to Africa be like a journey to the underworld (Hell)?
The purpose of a trip to the underworld is to go, get information, see things, and then live to tell the tale and more important, grow from the experience and go on living a better life. This requires honesty. That is what links Marlow and Kurtz: above all, they hate lies. About life, about the world around them and the people in it. Certainly the trip is arduous and the doings of the Europeans shocking and unpleasant. But one can find those things in other places, on other trips, even without leaving Europe (in contemporary 1900's Poland, for instance). But what makes this trip a special trip to the underworld is that it takes Marlow to a very different place and gives him perspective. The trip up the Congo River and back takes him out of himself and lets him vicariously live --and die-- as Kurtz, a remarkable man. On his return he 'becomes' Kurtz, with the important difference being that Marlow is alive and Kurtz, merely 'a voice'! The fact that the trip is also an existential trip 'into the mind' makes it a special Modernist journey to 'hell' as well.
If you consider the theme of moral corruption in the book "the Heart of Darkness" it becomes very clear that Marlow's journey to Africa could be like a journey to the underworld. Racism, madness, loneliness, disorder, corruption all exist within Marlow's journey, and take strongest hold upon Kurtz, who lets the evil influences take full rein upon his conscience. The savage nature of man is revealed, even uttered by Kurtz, "the horror, the horro." When there is no order, only lawlessness and immorality, it could be considered the underworld