Yes it might be a cache and not just a few bars. I am new in this field and is just starting to do some research. Your are right, this will be a difficult, expensive, and risky project. So I am doing it slowly and carefully. The areas are near previous Jap headquarters and are now utilize as paddy rice field. Interviews with old folks seems to be positive. There are lot of areas in here with such soil condition, and probably might be even deeper, for the Jap had use POW for digging. For now, I am looking for detector/locator (not so expensive) but should be functional in such conditions, for it will be very expesive to dig just anywhere.
John, thanks, I have found the website "Accurate Locators". Their "Accumeter Pro VI" and also the Fitzgeralds' "Mother Lode Locator" seems appropiate, I still checking.
Thanks, and continoue visiting this site for update.