dbear , RobRieman is right. Ask yourself how an object is any- more -hidden, whether it's 1 ft deep, versus 20, versus 50 ft. deep . As long as the top ground is covered back up, then what the heck difference does it make to go deeper still? Isn't the purpose of burying to hide it from others? Thus so long as the surface ground is returned to normal, then going deeper doesn't hide it any better. In fact, it makes no sense. I mean, doesn't the person who hid it, intend to come back for it some day ? Have you ever dug a hole 10 ft. deep manually ? Or, heck, 50 ft. ? You're talking all-day work !
I'll even go a step further and say that anytime someone comes on talking of "gold bars" at "3 to 5 meters" (20 to 50 ft) is gauranteed to be superstition, lore, ghost stories, and usually coming from a 3rd world country like Mexico of Phillippines, where 'sure-fired-treasures" are in every cave.