I personally dug out a 6x3x1 garden bed last summer, 90 degree heat getting annihilated by mosquitos, in about a little under or over an hour. My soil is just rocks and clay. Given this information, we can estimate that I could have flown there, dug the thing out, and left, in the span of a weekend.Three feet deep?
It's been five months, for crying out loud...you could have dug the Marianas Trench, the Great Lakes and the Dead Sea in that time...
Now going off of that time estimation and jjohn's example, in the span of 5 months, 150 days, working 8 hours a day, while I wouldn't be able to dig out the Marianas trench or any of the other crazy things, I'd be able to singlehandedly dig out 21,600 square feet, or a little under half of a football field. Just saying...