Do you know any neighbors, PB, anyone who may have knowledge of how the pool was filled in? Perhaps they could shed some light on what you are facing.
Last year we purchased a home with a filled in pool. We had originally hoped to dig it back out, but once we learned how they did it, we realized it would be a massive undertaking of both money and labor. (About 75% more than the cost of installing an in ground pool from scratch!) We've opted instead to partially bury an above ground pool in a different location. Disappointing, but I'm so very glad we learned this before we invested in the project!
Good luck! I hope you have a better outcome than we did - it would be a very fun project!