Changes

To start you must dig a reasonable distance above [[water]]. Keep digging in one corner until you "hit rock" or you do not have enough skill to continue digging. Once you hit rock in one corner you then must dig in all other corners of the tile until they have all hit rock. When you hit rock in all four corners the rock tile will be uncovered. If you lack the skill to dig in "such steep slopes", you could dig the nearby tiles down further as well, to reduce the slope to the corners you're working on.