Pipe extensions for greater depth. These were used to dig wells. The Seymour company sells extensions. I wonder if regular black iron pipe threading would fit.

Digging holes with these things is easy.  Hardest part is to remove rocks that are too big to roll past the blades into the bucket.  So my thinking is use it when possible,  and avoid some of the back strain of normal digging.
Now I've never used one of  these to dig past the 18" needed for a post hole.  So I might not like the weight of pulling up buckets of dirt attached to multiple  lengths of pipe.

Rock and mineral clubs purchase and share slab saws and other equipment.  Why not something like this?  A 2002 Bobcat 331E Mini Excavator with cab.  Asking price is $10,400 (I bet that 400 is there just to encourage offers of $10K).  Available via Craigslist in northern Mississippi, the current owner says everything works as it should, with good tracks and 3,060 hours on the clock.  If this is indeed the 331E model, the maximum digging depth is 12.8 feet.  Nice price, or leverite?