This is - believe it or not - a radish! Shumway's seed catalog sells this variety (the only place I can find them). One year I wanted to grow something unusual in the garden I got blue corn (no kidding - called "Blue Dent Corn"), white tomatoes, and giant black radishes. This one actually is not the largest from the garden. I have had them at least 7 inches in diameter. It is a little stronger taste than the normal radish - but I got tired of planting a whole row and using them in 1 week. Now I plant 8 radishes or so and get weeks of use from them! The amazing thing is these continue to thrive under snow (even here in the snow belt). They can be dug from the ground (frozen dirt permitting) mid winter, let them thaw for a day, cut them and eat them. They are just as good as when harvested in the fall. I like to take a thin slice of these and put them on a sandwich - covers the whole piece of bread! Somehow I cannot imagine going to restaurant and being asked if I would like a slice of radish with my burger - but I do it at home!