I've heard that a ring of stone, metal, or even of boards laid up, was placed on the ground and the wall was begun on that. The earth was slowly dug out from under the base, and the wall settled down into the well, and the wall continued from the top. I don't know if this has ever been verified, but it seems workable, provided I'm not the one down the hole. I'm claustrophobic.

Member, Timber Framers Guild