The 'optterrain' did not optimize so no change there. I decided I would wait until the area formed a crater to dig further around in the slim chance digging would remove the "bad" section. The first time I hit the bottom with a pickaxe, the surrounding ground shot up to be almost level with the ground level of my base. The depth is about two large stone walls high. But to make things even more confusing, the next time the crater appeared, the ground didn't pop back up so I dug around the area in all directions. Now I'm waiting to see what happens before I try filling in the hole.