I "have" dug down to a foot, but in this rocky soil we dig in around here, depending on how tired I am, the weather (how hot humid it is) the location or how good the tone sounds decides how deep I'll dig a particular target.
IN this rocky soil, in many instances if it's 10-12 inches deep, I may be inclined to walk on...not always, but in some instances, if I dont have the target after 8 inches, I'm not digging to china... besides that I have NEVER dug a deep coin past 9 inches around here in my county. Are they down there? I bet they are, but I am not gonna kill myself diggin for a "chance" it's not just another piece of a deep rusty bucket like all the others were.
With the soil being so rocky, my train of thought is that coins probably don't sink past the rock layer anyway.
In general, if the target indicates silver I'll go to town diggin that is if the hunidity isn't at 324% like it usually is...lol