I've always been surprised at the amount of people that ask if the large burls were underground. It could be words like "unearthed" that has been written more than once that have caused this misconception. The large burls on the right hand side were actually sitting up on roots that were completely exposed. The left side was sitting right at the ground level, the roots were hidden under the ground on that side.
My guess is that the tree was nearly uprooted by a natural cause a couple hundred years ago when it was very young. It leaned over but after a few years of struggling to survive it straightened itself out. This 'wound' caused the tree to strengthen itself by twisting and curling into truss-like shapes which was the beginning of the gigantic burls. Nature is absolutely amazing at taking care of itself, and this piece of wood is one of the worlds wonders that shows that well.
This picture shows what the stump looked like after I had dug around the roots on the outside as well as the inside.