Children Who Chase Lost Voices (2011)

Asuna, a lonely girl, finds solace in the enchanting melodies of a crystal radio gifted by her deceased father. Her life takes a dramatic turn when a terrifying monster attacks her, only to be saved by a mysterious boy named Shun, who vanishes just as quickly. Determined to reunite with him, Asuna teams up with her teacher, Mr. Morisaki, and ventures into the mystical realm of Agartha. Along the way, she uncovers profound truths about the world's beauty and cruelty, as well as the bittersweet nature of loss.