There is another mechanism that is much more apparent in ground planted bulbs than potted ones, although the longer the bulb is in the pot, the more it becomes a factor. This is simply the breakdown and disappearance of the oldest organic components of the soil, usually at the bottom of the organic layer(s). The bulb naturally settles, and is buried deeper by the seasonal leaf and litter drop (or mulch). I've noticed this in action here in the northwest in Trillium, Erythronium, Lilium and many others. I would suspect this to be the major factor in most of those cases where the bulb is found sitting on the top of the first layer of soil devoid of organic matter. Of course there are short cuts that the plants take; just as John described, I've had Camassia seedlings form a new bulb three inches below the old one. Dave Brastow