Yeah, that I understand. But it sounds like, as you go deeper & deeper the gravitational pull is rather easy to calculate: It's directly proportional to your new distance from the center. (Ignoring the real-world problem of mass concentrations along the way, melting calculators, air-conditioner failing, attacks by the mole people, etc.)
You take Pat Boone along so he can fight the mole people. At least, from somewhere I remember something like that.