Journey to the Center of the Earth, published in 1864, is an early example of a science fiction adventure novel.
The adventure novel is one that is filled with excitement and danger with the action taking place outside of the usual daily humdrum of life.
Science fiction as a genre is speculative fiction that is usually based on technology or knowledge that does not yet exist or is not yet fully developed in the real world. Science fiction is different from fantasy in that the technology is often explained rather than simply magical.
Journey to the Center of the Earth is an adventure novel because Professor Lidenbrock, Axel, and Hans go on a dangerous and exciting journey to the center of the earth, a place few have ever been to. As readers, we are kept on the edge of our seats, because we don't know what is going to happen next. Verne delivers on the promise of the adventure genre by having his characters face obstacles and find such wonders as a giant mushroom forest, a sea, and humanoid creatures in the world underneath the earth's crust.
The novel also delivers as an example of science fiction, the genre for which it is chiefly known. It brings us, if not to a new planet, to a new world within our own planet. Furthermore, Verne stays with the known parameters of the science of his time, providing a careful scientific explanation for the phenomena in the earth's center, such as how it receives light. The thorough preparations the professor makes for the journey are also rational and scientific, with the characters, for example, carrying early prototypes of the flashlight, which had not yet been invented, to light their way.