I plant a garden every year and have tinkered with a greenhouse, but recently have become fascinated with the idea of a pit greenhouse. With the proper setup, you can grow year round, even here in Texas. I am very interested in the idea of cooling using underground ducting as well as a chimney as a draw mechanism. There is so much information on the web, but most of it is for Northern climates with not nearly as much information on year round greenhouses in the South where cooling will be more important than heating. Not to mention that so much of it is over my head. I seem to have the bug, though, so will soldier on.
Pit greenhouses can be effective ,even in texas.
Keep ventilation , air convection ,positioning with prevailing winds, and the use of 'shade cloth' in mind.
Also, given the temperature extremes, see what information is avialable through Cooperative Extension or the Farm Bureau.
They might have already assisted other pit greehouses in getting established ; there is no better information other than from someone who already has the hands on expierience.
They could also advise you in selecting crop varieties that would be more successful in a pit greenhouse envireonment.