How deep is 3,000 square feet?

This is a bit of a weird one. As part of my ongoing quest to become the ultimate dwarf, I've commissioned a bit of magical research that should be useful both for further mining and for dungeon exploration: A spell that transforms a quantity of unworked stone into a presumably-confused elf, who can then be more easily ordered out of the way than the unworked stone would be.

The Player's Companion seems to support this fairly well, with the desired effect and target being readily available in the tables relevant to designing custom spells... Except that, to my dwarven mind, "3,000 square feet of unworked stone" is strangly a two-dimensional target. If my pet mage were to cast this spell on an exposed area of unworked bedrock, what depth of stone would be affected? It seems absurd to affect all the stone in the area right down to the mantle, and yet it seems almost as strange to say that only a paper-thin slice of stone would be affected.

Since I have no immediate desire to create a new volcano on my own land, I am forced to ask rather than just trying it out: What exactly does my spell do?

The spell which uses the 3,000 square feet of stone targeting is Transmute Rock to Mud. Looking up Transmute Rock to Mud, we see that it affects a depth of at most 10’.

So I am not an Autarch, but that would be my answer, a chunk of unworked stone that has an area of 3,000 square feet affected to a maximum 10’ depth (which may or may not reach the full 10’ along the entire area, but will max out at 30,000 cubic feet. A wall 5’ wide with buttresses 5’ back every 100’, for example, would not transmute the full 30,000 cubic feet, because the wall is not 10’ deep the whole way.)

Right. It's 30,000 cubic feet, with no more than 10 feet of depth.