A Journey to the Centre of the Earth is a first-person account of an extraordinary voyage. Axel, the narrator, is a youth sent to study science with his multitalented uncle, Otto Lidenbrock; he is present when the professor finds a manuscript hidden in an old copy of the Icelandic epic Heimskringla. The manuscript contains an encrypted message signed by the celebrated alchemist Arne Saknussemm, which purports to give directions for reaching the center of the earth. Having solved the cryptogram, the professor immediately resolves to follow the directions and the narrator goes with him to Iceland; their party is eventually completed by an Icelander hired as a guide.

The narrator describes in great detail the equipment assembled by the professor. Their discussions regarding the usefulness of various items provide a running commentary on all that was then known about the structure of the earth’s crust. Axel also describes the professor’s eccentric but methodical attempt to offer him some relevant training by making him climb a tall tower, in order that he might overcome his tendency to vertigo. The descent into the extinct volcano is described with equal scrupulousness, with a full report of the scientific observations made by the professor. At first, the temperature increases the further down they go (as contemporary theory had predicted), but, as the story moves into purely hypothetical realms, it reverts to a more comfortable level.

Deep beneath the surface, the three explorers enter a vast, illuminated cave system that seemingly extends beneath the whole of Europe, and perhaps the whole of the world. It harbors many strange life-forms, including some that once existed on the surface but that have long been extinct. They find a subterranean ocean and set about building a raft on which they might cross it, heading south-eastward toward the region beneath continental Europe. They are menaced by a plesiosaur and are hastened on their way by turbulent weather, eventually reaching a further shore. Amid a veritable treasure-trove of fossils, they discover a human skull—and then, in a primeval forest, they catch a brief glimpse of a giant humanoid patiently watching over a herd of mastodons.

After discovering more signs of Saknussemm’s passage, the three explorers begin to make plans for their homeward journey, but their progress is interrupted by a huge lump of rock. Their attempt to clear a path with explosives causes more disruption than they intended, and their raft is swept away, eventually being forced up a narrow shaft that the professor recognizes as the core of another volcano. The volcano’s eruption hurls them out onto the surface of the earth, delivering them to safety on a tide of lava. They discover that they have been ejected from the mouth of Stromboli in southern Italy.