Potatoes - how deep?

In the same way tomatoes will root on the stem if it is buried, so will potatoes. Potatoes form on underground stems. Earthing up (or planting deep) increases the length of stem underground, so increases the opportunity for potatoes to develop. It must increase the yield or farmers would not use expensive ridge making machines when planting spuds. Planting extra deep is OK, but the plant has to use much more energy before leaves reach the surface and replace the energy used.