Hoshi wo Ou Kodomo: Agartha no Shounen

Shin lives in the underground world of Agartha with his older brother, Shun. After Shun leaves for the surface carrying a crystal, Shin decides to follow him and retrieve the crystal for Agartha. On his journey, he encounters Asuna, a girl who has no father, and Morisaki, a man who has lost his wife.