The Oya stone mine can be found near Utsunomiya city in Tochigi prefecture Japan. The mine is now a museum and can be viewed by simply walking into the building and paying a fee.
Some items are on display in the building plus photos of the old days of the mining industry there. Then you can take your time exploring the mine itself. Walking down into it is not too dificult and just requires you to negotiate some stairs before you are met with a great view of the inside of a hill and a mine.
The Oya stone is a stone that was mined for the purposes of building and many buildings in the area can be found made from the stone - the Utsunomiya Catholic church is just one example. The Oya Peace kannon is just a short walk from the mine.
The mine can be reached by bus in about half an hour.
The Oya mine museum where the entrance to the mine is.