There are innumerable locations impossible to dig,especially were the majority of decent terriers breeds evolved,thats possibly why a sounder that could bolt was prized over any other working terrier type,back in the day when terriers were grafted harder,longer and with more purpose than the majority are today,digging was the easy option brought about by the advent of technology,since technology started to play a major part in terrier work,terriers have gone backwards,get a mark and get the shovels out lads,breed them bigger,stronger,muter and less useful,the shovel and locator will amend the