Underground homes had a brief following in the early 80's - the golden age of passive solar construction. The primary advantage of such structures is the relatively constant temperature of the soil - about 50F. This can be an advantage in very cold and very warm climates.

The problems with such structures are:

Higher construction costs (and lower oil prices).

The need for quality waterproofing and drainage systems.

Interior air circulation systems to prevent mold.

As a result of these findings, many people opted for more conventional structures such as those with: high insulation levels, passive solar heating and/or cooling, and ground coupled heat pumps.

Due to today's lower level of interest in underground structures the 'net' is not likely to have too much information. I would suggest that you look at the material available in print, for example, look at back issues of Fine Homebuilding.

Good luck and let us know how things are going. --George