We made a potato tower two years ago. We planted it with seed potatoes, added soil as the plants grew, and then proceeded to forget about harvesting them.
This year the plants started to grow again, so we decided to leave them alone and see what happened. The potatoes from last year continued to grow and when the plants died back we harvested our crop! I can only imagine how many we would have gotten, had we done a better job of tending to them. :)
If you want to make your own potato tower, follow this link: Making a Stackable Potato Tower