This has been talked about in another thread; someone called them "lazy man's 'taters", but it works:
I salvage every bale of hay I find on the side of the road; they're mostly round bales that fall off a truck/trailer and break. Most folks don't bother to come back for them. I get them when I have a chance, pitchfork at a time in the bed of my truck, pile them in an out-of-the-way place in the yard and let them rot for mulch and compost.
Potatoes will grow in the piles if you keep them watered. My mom harvests them mostly as "new potatoes", but they will mature if the pile is more than a foot or so deep...and kept watered.
This will also work in leaf piles.