Letters and mail from around the world, for which the addressee or intended recipient is unknown, are sent to the Hyoryu (which means adrift) Post Office on Awashima Island. Here the Hyoryu Post Office carefully takes care of postal items addressed to your past self, to your precious lost loved one, to your future children, to that an unknown person, or even addressed directly to the Hyoryu Post Office. 
*"Drifting Post Office" is an art project by contemporary artist Saya Kubota, using a 50-year-old post office, and is not affiliated with Japan Post. (It is not a real post office.)