I own this digger and have used it for years, i also own my grandfathers digger which he had for over 40 years. This is by far one of the best hand hole diggers i have ever owned (i used to own a fence company and used this when digging near underground utilities) anyway you can add extension pipe to this digger and dig as deep as you want as long as you can pull up the weight. We ordered a new 4" head for it and have dug 18' with it once at a friends camp. all you do is remove the head add 3 or 4 feet of pipe put the head back on and continue to turn till you need to add pipe.
Good luck and ask any questions
Adjustable Steel Auger by Seymour Mfg Co - More Post hole diggers at doitbest.com