Following the course of the Hansbach, the explorers descend many miles and reach a cavern of colossal size. It's a genuine underground world that's lit by electrically charged gas near its ceiling, is filled by a deep subterranean ocean, and surrounded by a rocky coastline that's covered with petrified tree trunks, the fossils of prehistoric mammals, and gigantic living mushrooms. The travelers build a raft out of semipetrified wood and set sail. The professor names the ocean the "Lidenbrock Sea" and their takeoff point "Port Gräuben", after his goddaughter back home (whom Axel will marry at the novel's end). While at sea they encounter prehistoric fish from the Devonian Period and giant marine reptiles from the age of dinosaurs, including an ichthyosaurus, which battles and defeats a plesiosaurus. After the conflict between these monsters, the party reaches an islet with a huge geyser, which Lidenbrock names "Axel Island".