(French: Voyage au centre de la Terre, also translated under the titles Journey to the Centre of the Earth and A Journey to the Interior of the Earth) is a classic 1864 science fiction novel by Jules Verne. 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 descend into an extinct Icelandic volcano, encountering many adventures, including prehistoric animals and natural hazards, before eventually coming to the surface again in southern Italy.
From a scientific point of view, this story has not aged quite as well as other Verne stories, since most of his ideas about what the interior of the Earth contains have since been soundly refuted; however, a redeeming point to the story is Verne's own belief, told within the novel from the viewpoint of a character, that the inside of the Earth does indeed differ from that which the characters anticipate. One of Verne's main ideas with his stories was also to educate the readers, and by placing the different extinct creatures the characters meet in their correct geological era, he is able to show how the world looked a long time ago, stretching from the ice age to the dinosaurs.
That's the fiction, now, if you want to know what is really going on below the rocky surface you lay your feet, watch this short BBC documentary called: Earth's Core.