Hoshi o Ou Kodomo - Agartha no Shounen

A young man named Shin lives with his big brother Shun in an underground place named Agartha. One day, his brother left toward the surface with a crystal and Shin had to follow him and bring back the crystal to Agartha. During his journey, he will meet Asuna, a young girl who lives without her father and Morisaki, a man who has lost his wife.