> To achieve comparable volume of the rectangular prism, you will need to dig a much deeper cone.
Yeah, holes with sloped sides are easier to dig than vertical-walled ones. But you don't need to have them sloping all the way down to a deep point in the middle; for a shallow (comparatively, like this) hole you can just slope the walls down to the same flat bottom.
At a 45-degree angle, you could have the bottom of your 100-yard-deep pit be 35 miles - 100 yards across, and the opening 35 miles + 100 yards. Which would be a pretty negligible difference on the surface -- and, I think, make the volume a tiny bit bigger.
Yeah, holes with sloped sides are easier to dig than vertical-walled ones. But you don't need to have them sloping all the way down to a deep point in the middle; for a shallow (comparatively, like this) hole you can just slope the walls down to the same flat bottom.
At a 45-degree angle, you could have the bottom of your 100-yard-deep pit be 35 miles - 100 yards across, and the opening 35 miles + 100 yards. Which would be a pretty negligible difference on the surface -- and, I think, make the volume a tiny bit bigger.