I am glad you are consulting an architect or engineer before you under-mine your footers, but I just can't figure out why you would to the effort of digging out this space that is covered with 4" concrete and is probably just fine for storage. For every foot of material you remove, you will generate over 16 cubic yards of spoils, enough to fill a full size tandem axel dump truck. And for what? The house will not increase in value and the space will be of limited additional use. If you need more space its much easier to go up or out.
I applaud your ambition Richard, but I think you are nuts. This mining operation is incredibly difficult. How are you going to move that much broken concrete, dirt and rock out of the basement to where you can haul it away or dispose of it. Anyway, no basement is worth clutching your chest over.
I encourage you to consider the higher value and return on investment of going up or out with an addition. Good luck.