I've been building mine with lots of thermal mass. Not dug into the ground, but thick walls and outsulation.
I'm in the Hill Country near Bandera and Utopia.
I don't seem to see any references to Penn & Cord on this thread so here it is: Penn & Cord
They design some of the best passive GHs around and are easily home built. Here is one of their beauties:
They heat and cool passively and could easily be changed a bit to accommodate an AP system. Personally I would just add the fish room on the back as in our design. A must see site if you are contemplating a GH and want to keep heating and cooling under control.
We're in Arizona, and this year I'm trying to grow loofa as a shade screen instead, or in addition, to the regular ones we put up every summer.
Has anyone played around with icyballs? They were used in the early iceboxes and used hot water to cool the fridge. In the hot areas, solar heat could heat one ball, and the other could be placed in front of a fan, thus using the sun outside to cool the air inside without adding moisture.