* Draw outlines around the cells based on their depth, so it's easier to see how deep they are.

* Add something like a "mining machine" which must be at level 0, and makes a path of steps of 1 height.
  The aim is to have a way of getting money once you're stuck, and to give an incentive to not dig some cells.

* An expensive "just dig this cell no matter what" tool?

* Add a popup thing each time you make a move, showing how much money you dug up or spent.

* Different kinds of treasure, to make it interesting.