As Marlow is traveling upriver to see Kurtz, he is very uncomfortable in his surroundings. They are foreign to him, even though they probably feel quite natural to the native population. He has 20 cannibals as his crew and the massive trees make Marlow feel very small. At this point he says, "the earth appeared unearthly". In other words, it was unnatural, eerie and ominous for Marlow. One must also remember that Marlow is recounting this story from memory and memories often are hazy. So Marlow focuses on his emotions, which he remembers better than the actual scenery. At this point, he is frightened and says even the men appeared monstrous. Marlow is truly out of his element and facing an unknown situation and thus nothing he remembers appears "real" because he is so unfamiliar with it.