deeper underground structures

yeah I have a question, did Mike ever try digging deeper into the earth?  sometimes I think about how cool it'd be to dig downward, you could do sort of a mansion, even a skyscraper..., but it instead of outward and upward, it'd be down.  lol. 

Cave house for sale on Ebay last year

http://damncoolpics.blogspot.com/2009/02/cave-house-for-sale-on-ebay.html

well I posted a question in the wrong place I guess.  somehow it dissappered but I'll ask again:  you ever dig deeper into the earth Mr. Oehler?  the concept designs found on this site are tempting for me to build.  Somewhere it was written that these are three times easier to build than cob, but if you include the steps of approval, AND the labor, i'd say this method is more than 3 times easier.  probably more like 10-20, depending on your area.  Anyway, I bought your book at borders, synchronistically, a couple of years ago.  I think everyone who sees it on my bookshelf, probably sets their disposition that I am crazy in stone, but the feeling is mutual in that respect so it's still all good ; ) right??

My question again is, you ever dig deeper into the earth?  what kind of problems would you run into going down deeper than where you went to.