The Tunnel to Summer, the Exit of Goodbyes: Ultramarine (Manga) Vol. 1

Kaoru heard a rumor: The laws of space and time mean nothing to the Urashima Tunnel. If you find it, walk through and your heart's desire will be on the other side...in exchange for years of your own life. The new transfer student Anzu promises to help Kaoru experiment with the mysterious tunnel--but what does she want from Kaoru in exchange? And what will he have left to give, after the tunnel's done with him?