You allways hear and read " oh yea just bury it" right? To dig below ground storage is a lot of work. I do not personally own a back hoe and many of us don't. A shovel is really hard work especially if the ground is hard. An idea came to mind and worked very well which was using a post hole digger, mine has a 4" round x 24" long and I purchased a 12" extension. Now a good storage would be starting with a 4 foot square and within that grid dig the first corners and maybe a couple of center holes and then finally the last two corners. This leaves a ground site that is even depth and the shovel work is much easier at that point. If a deeper storage is needed then repeat within the 4 foot hole digging say a 3 foot square area and then more shovel work. This sounds like a lot of trouble but try just doing this with a shovel by itself. According to the shape, or amount of bulk to be stored depends on the widths and depths. Don't forget the top of your items to the ground level measurement because it allways seems you need a deeper hole. I will not dwell on how one might map the below ground storage sites but maybe a standard red brick standing on end buried where they can just be seen or easily located by scraping some dirt away can mark the diameters of the site area dug because to dig up and retreive the supplies that were initially buried you would have to start digging a little outside the perimeter of the site o say about 10-12 inches outside the border. I would think that the procedure for unearthing the goods is as important placing them there. There are many ways of packaging items for ground storage and I guess that is a whole different thread...subject. Take my word on this one from experience that the post hole digger does a great deal of the work. Wear ear plugs because the engine is more near your ears than say the lawn mower.