Stick a piece of PVC pipe in the ground and start a seperate thread about the mystery of why it's there. Half of ODT will show up with shovels to dig it up.
Surprised the ground is that hard there as it doesn't appear to be red-clay type soil. I would rent a rototiller and go over the 8 x 8 area to loosen the soil to two or three inches, rake or shovel it out and fill with water and let it set overnight, repeat and see if it doesn't loosen the soil enough to use the rototiller. May have to do this two or three times. I helped my son dig a hole about 12 miles north of you in red clay to set out a 12 foot tree - tough go.