to just fill it in break holes in the bottem so water doue not pool lol,
and fill, if you leave it empty it could float out like a big boat,
To remove you need and excavator = rock hammer, but they will just leave the material in and fill it over bit like rocks,
easiest way is to contact a pool company as they have lots of fill they need to get rid of and they pay when it goes to the tip, and their bobcat can fill it.