とかいいつつ、この手のは海外でもよく見るので、人間の性質なのかもしれません(笑)
もしくはこの鉱山からは硬貨が出土してたとか！

Despite what I say here, I see this a lot even overseas, so maybe it's just human nature! :-) Or maybe they're excavating hard currency from this mine!