Professor Hardwigg, a learned geologist, discovers an old Runic manuscript written by the sixteenth-century traveler Arne Saknussemm. The text claims a path through the crater of the extinct Icelandic volcano Sneffels leads directly to the earth's core. Despite the initial hesitation of his nephew Harry, the professor resolves to undertake the perilous descent. Accompanied by a taciturn Icelander named Hans, they venture into the dark volcanic shafts, descending thousands of feet below the surface.
Their journey leads them through vast subterranean galleries, coal mines of the early earth epochs, and endless tunnels filled with danger and mystery. After enduring severe thirst and becoming temporarily separated, Harry is reunited with his companions near a subterranean stream. They eventually discover a vast, luminous underground sea complete with tides, storms, and shores covered in antediluvian flora and the fossilized remains of prehistoric beasts. Using fossil wood, Hans constructs a raft to cross this mysterious ocean. As they sail across the subterranean waters, they encounter colossal marine monsters, including battling ichthyosaurs and plesiosaurs, testing their courage and resolve as they push deeper into the uncharted interior of the globe.
