Jules Verne believed in writing stories (the term science fiction didn’t exist yet) that were based on real science, as understood at the time of his writing. Many of his stories seem ridiculous today because of what we know now. For example, 20,000 leagues under the sea would be on the far side of the moon on the other side of the Earth. However, when he wrote the stories they were consistent with accepted scientific knowledge.
H. G. Wells wrote stories about things that came straight from his imagination. Famously when H. G. Wells wrote The Time Machine Verne accused him of ‘making things up.’ Time machines, invisible men, animal genes spliced with human, are all things that are not even scientific ideas so much as just ideas around which to tell a story and comment on the choices and futures of humans.
Today those two schools of thought have evolved to be called hard science fiction and soft science fiction. Hard science fiction is fiction that poses future tech or science that is possible according to what we know currently of science (physics, biology, etc.). Soft science fiction would be things like Star Trek where the future science isn’t intended to seem real, it’s intended to give the authors a medium by which to express a metaphor.
Journey to the Center of the Earth is a fine adventure that comes across as rather silly in the light of today’s knowledge of the composition of the earth. When it was written, it was part of an ongoing debate about the causes of molten eruptions and what might be beneath the surface of the earth.
It is simultaneously a seemingly silly notion but also treated much more realistically and seriously than any of the various motion picture adaptations.
Jules Verne is eminently readable and mixes his explorations of scientific debate and theory with genuine humor and astonishing adventure with great skill.
You owe it to yourself to read some Verne. If expeditions through dark tunnels in the subterranean world seem too claustrophobic to you then try a trip through the depths of the ocean with Captain Nemo or fly a balloon around the world in 80 days. Whichever one you pick you will be treated to a great adventure that requires only a little suspension of disbelief as you readjust your sensibilities to the late nineteenth century.