When I was a teenager, the first Chia pets were introduced. They were one of the big fads of the eighties, along with giant plastic combs, painter's pants, and pet rocks. So when my neighbor looked like he was growing the world's biggest chia pet in his backyard, I asked him what was up.
As it turns out, it's a vertical potato cage, built similarly to this one. The idea is to build a big circular cage, fill it with compost and dirt, and grow potatos vertically in a small fraction of the space needed to grow them in rows. According to Fred's Garden Gate, you can grow half a bushel a for a horizontal foot.
I'm hoping that maybe it also works with yams, since we don't really eat many potatoes. I'll let you know how my neighbor's Chia harvest goes!