Single mother and brand new above ground pool owner... Install completed yesterday however there were some issues when digging. First of all I had a 4-ft fall that needed addressed. Then while digging they hit an enormous boulder. So they decided to move their original area back a couple of feet. So now I am left with this huge gap and hole between the side walls and the ground. Need some major advice. Can I back fill this enormous area? And if so do I need to do a retaining wall inside as well? My original plan was to do a step down addition to my current deck. However I don't want to just leave this hole open even if its underneath a deck. We get a lot of rain and I would hate for it to accumulate water. Pictures attached... All advice welcome... Thank You!!!