Yokohama Station SF, Vol. 1 (manga)

All Hiroto has ever known is a life on a tiny coastal speck of Japan. Much of the country has been swallowed by Yokohama Station, a mysterious, ever-growing series of buildings that's been around for as long as anyone can remember. The few who live outside its many entrances have never seen Inside and know only rumors and legends of the station's interior. That all changes when Hiroto is given an 18 Ticket, a mysterious item that lets him enter the massive complex for five days. The young man has always sought a purpose, but the one he finds may not be the sort he'd hoped for...