Pluton no Musukotachi

Synopsis

Rieko is the grand-daughter of Eisaku Shirakawa, a scientist who, during the World-War 2, worked on a top secret project about the Hollow Earth theory, sponsored by the Nazi government. Rieko embark on the Yamato, a probe drilling vessel, to see Ryuu, her brother working on it. Despite being docked, the Yamato suddenly starts drilling the ground more and more, without Ryuu nor his colleague being able to control it. The three of them collapse due to the temperature. When they finally wake up, they discover that they have reached the Hollow Earth!