The opal mining town of Coober Pedy has dwellings mined into the ground. Sure, it excludes sunlight, the 100% exclusion factor, but it's also known that getting down under the ground is a great insulator.
Actually it provides thermal mass, not so much insulation. The day-night temperature variation is high; earth-bermed or fully dug in housing (or any high thermal mass construction) smooths this out as the heat absorbed during the day is radiated back into the house overnight.