The Tunnel to Summer, the Exit of Goodbyes (Light Novel‪)‬

One summer morning before school, Kaoru hears an unsettling rumor--of a mysterious tunnel that can grant any wish to those who enter it, but ages them dramatically in exchange. At first, he writes it off as nothing more than an urban legend, but that very night, he happens upon the selfsame passage: the Urashima Tunnel. As he stands before its gaping maw, a thought occurs to him--if this tunnel truly does have the power to grant any wish, could he use it to bring his younger sister back from her untimely death five years prior? Yet when he returns to explore the tunnel the next day, he finds he's been followed by the new girl in class: a total enigma by the name of Anzu. She takes an interest in Kaoru, and they agree to work together to investigate the time-twisting tunnel and uncover its mysteries. Together, they might achieve their deepest desires...but are they prepared for what it may cost them?