Hoshi o Ou Kodomo - Children who Chase Lost Voices from Deep Below

The story centers on Asuna, a girl who spends her solitary days listening to the mysterious music emanating from the crystal radio she received from her father as a memento. However, one day she meets a mysterious boy and is thrust into a magical world beneath the earth. This is their journey to find the Quetzalcoatl and the reasons behind life and death.