Building underground has many advantages. The most obvious is insulation. For those who live in the mid-west: There is the added benefit that your house will not get blown away. There are some excellent resources available for those who wish to build underground: http://www.williamlishman.com/underground.htm If your looking for something that lasts 1000 yrs - subterranean is your best bet. Here is another great example of a subterranean house: http://www.homesandcottages.com/ExtremeHomes/tabid/138/Default.aspx?ArticleId=168#

Actually the Igloo as a subterranean structure is very sound. Having the structure underground also provides other heating and cooling benefits. There is some excellent underground architecture out there: http://www.williamlishman.com/underground.htm