Gunkanjima or 'Battleship Island' was once the mostly densely populated placed on earth. Located just off the coast of Nagasaki, Japan.
A golden button will reward your efforts.
The cache itself cannot be found by visiting the island by way of a regular tour. Explorers will need to visit with permission from the city government to access the part of the island which contains the cache. The coordinates are exact to the building and rough area inside and the cache is located in a small black pouch. All you need to do is discover which floor :).
Ba n furys ng rlr yriry va n ebbz arne gur fgnvepnfr.