The leper in the wilds of the wasteland is a symbol of the uncivilized, primitive, and hostile world in which the men are venturing. Such a man is indeed an ominous sign.
Light and Dark (Motif)
The novel uses the motif of light and dark to create mood and suspense. Light, particularly that of lamps and the eerie electric light infusing the cavern, provides guidance and a feeling of safety. Darkness is terrifying and lonesome, as when Axel is lost alone and his light breaks.
The Storm (Symbol)
The storm is a symbol of the fantastic power of nature and of how Lidenbrock cannot control natural events, even if he wishes to do so.
Water can be a source of sustenance (the brook) or of danger (the sea). The conflicting interpretations are appropriate given the fact that Nature is both benevolent and overpowering, depending on the circumstances.
The Unconscious Self (Allegory)
"The most common interpretation of the book is that it is an allegory for going deep within the unconscious self. Within this line of thought, there are certain conclusions which can be made about the human psyche according to Verne. The way to the center of the Earth was revealed when, after several days of cloud cover, the sun appeared from behind them to shed light upon the situation, perhaps a metaphor for the difficulty of gaining access to the unconscious self. Then, as the men made their way deeper below the surface, danger upon danger was thrown at them, and only deep thought and decisive action could overcome these obstacles. Finally coming to conclusions about the beginning of Man (which can be read as the origin of the self) they are expelled from the most inner reaches of Earth and escape as better men" (Onlineqrlab.com).
Journey to the Center of the Earth Questions and Answers
The Question and Answer section for Journey to the Center of the Earth is a great
resource to ask questions, find answers, and discuss the novel.