A balance must be met though, the footprint of the underground unit must exceed 30 square meters, so an increase in wall thickness will eat into interior space.
With this in mind, and backed by research, I have made the decision to increase my wall thickness for the underground unit 500mm. This is an increase of 60% over the other exterior walls, which are built at 300mm thick.
http://answers.yahoo.com/question/index?qid=20081217185558AAN3WPQ
http://www.supaloc.com.au/systems/framing