I found these while I was planting some mangels tonight. They have a very thin skin. the smallest is about pea sized. They're pure white when you cut them in half. They don't have a lot of smell that gets past the dirt smell. I am in east central Ohio.
I am in zone 6/7 and every tuber has already sported by now here but it seems that those have not yet.
I have found potatoes from the year before and I actually have quite a few that have "re-seed" themselves.
SO they could be japanese/chinese yams, potatoes, ground nut.
Maybe others will have other suggestions
My first thought was baby puffball mushrooms but you didn't say what their consistency was, hard or soft or...? Is it a root like a potato? I would rule out groundnut, which are connected by thin rhizomes in a chain-like fashion.
They are very firm, less porous and less water content than a potato. They were underground and came up when I tilled for the mangels. They are very bright white when cleaned off. They don't have any type of noxious smell, and there wasn't anything else attached to them.