Answer:
The answer is ... the mantel I think! But I may be wrong and it may be the outer core ... although that is just surrounding the inner core.
Step-by-step explanation:
The mantle is the part right between the thin crust and the hot outer core. So, if you look at a "map" of the inside of earth - the mantle is probably the one that you're going to be stuck digging through the longest. I'm sorry I couldn't explain it right, but I tried ... don't be mad if this is wrong - I'm just taking my best guess!
Have a good day.
-Lia.