Chou-Wakusei Senki : MetaFight

The classic story of a boy and his frog who runs away, becomes radioactive and jumps into a hole. More importantly however, it's the story about a boy who finds a fully functional battletank, SOFIA the 3rd, and goes adventuring around the underground. His mission... to find new weapons and add-on parts to the battletank which will in turn allow him to explore more areas until eventually he finds that freedom-loving mutant frog. And while he's down there he might as well defeat the Evil Plutonium Boss who wishes to threaten the Earth.