In the World of Makoto Shinkai - "Children who Chase Lost Voices from Deep Below"

Storyline

Asuna is a young girl who has been forced to grow-up quickly due to the death of her father and spend her days listening to mysterious music from a crystal radio which she received as a memento. On the way to her favourite mountain hideout, she is attacked by a fearsome creature and later saved by a young man named Shun, who came from a faraway land called Agartha.
They begin to open their hearts towards each other and Asuna promises to be back, but is not able to find Shun the next day. In school, a lecture about the mythological underworld Agartha, the land of the dead by substitute teacher Morisaki catches Asuna's attention.
Asuna then comes across a boy that resembles her saviour at her hideout, and eventually finds out it is his brother Shin, who is being chased by a group of men after his Clavis, the key to Agartha.
With the entrance to Agartha before her eyes, Asuna makes up her mind to see Shun once again and together, they set out on a journey into a land of legends…