From librivox.org:

The story involves a German professor (Otto Lidenbrock in the original French, Professor Von Hardwigg in the most common English translation) who believes there are volcanic tubes going toward the center of the Earth. He, his nephew Axel (Harry), and their guide Hans encounter many adventures, including prehistoric animals and natural hazards, eventually coming to the surface again in southern Italy.

From epubbooks.com:

As irascible scholar Professor Lidenbrock pores over a rare Icelandic tome, he discovers a scrap of parchment with cryptic writing tucked away between the ancient pages. And when his nephew, Axel, finally breaks the writing’s secret code, he learns of a hidden underground passageway that may lead deep into the center of the earth. Despite Axel’s misgivings, he and the obsessed Lidenbrock travel to Iceland and, with a guide named Hans, set out on a perilous expedition in the course of which the trio will encounter an extraordinary new world of extinct yet living species, an underground sea, and gigantic, battling monsters.Filled with the authentic detail and startling immediacy Jules Verne labored to bring to 20,000 Leagues Under the Sea and Around the World in Eighty Days, Journey to the Center of the Earth is the fantastic adventure that secured Verne’s reputation as the premier writer of speculative fiction.