Bricks, Masonry, Asphalt and Concrete - Basement Walls and Floor

I have an old dirt cellar that was dug out from under our house long after the house was built. It's a fairly small area, {Haven't measured it exactly yet} maybe 8 foot wide by 10 -12 foot long, 7-8 foot deep. I know already that I will have to install permanent jackposts under the footers if I leave it as it is. With the sudden escalation in really bad weather here that requires being underground, I am considering putting walls and a floor in the cellar instead of the jackposts. What is the best way to do this in an area that has already been dug out from under the house? Which would be best to use for the walls, poured concrete or block?Thank you.

This project seems to border on structural changes, in that you are modifying the footers. It seems prudent to hire a structural engineer for guidance in this matter. Having a footer fail due to modification could have unpleasant consequences.