Apparatus and method for supporting a post. In accordance with some embodiments, a subterranean support structure has a tube extension adapted for placement within a hole to receivingly nest a proximal end of a vertically extending post that extends above the hole. The tube extension has opposing first and second ends, the first end adapted for placement proximate a top elevation of the hole and the second end adapted for placement proximate a bottom elevation of the hole. A support flange circumferentially extends from the tube extension a selected distance from the second end of the tube extension. The support flange supports a column of anchoring material placed in the hole between the tube extension and a sidewall of the hole.