I hope I haven't missed someone else saying this but I think Diana is hinting at a view I have for many bulbs observed in the wild. Don't forget earthworms etc - if a bulb holds it self fast by it's roots and even pulls a bit - as some clearly do - then, as the burrowers works around, it will tend to collapse the tunnels under itself and soil will be moved in above as the various creatures go about their business. I am sure this is something the wildlife time-lapse experts could - maybe have - capture. Then, for woodland bulbs at least, and to some extent all, there is the continual renewal of the soil structure from above from leaf litter as it gradually rots below which will carry bulbs down over the seasons. Reply / forward from John Crellin Maybe the focus is too much on the bulb.