I have used quite a few detectors and have found some really deep beach finds................
The deepest coin I ever recovered on land was in the woods under an evergreen; A silver quarter down right at a (very honest) 20 inches.
Now it was in mostly decomposed nettles, and BARELY registered (ziiiip, ziiip) with my Fisher 1266, which I had cranked to the max, sensitivity pushed in
and full clockwise.
I almost passed it up, but if any of y'all have ever used a '65/66, then you know the zip I am referring to and it nearly always means coin.
By the way, it was a fairly worn 1945 quarter, and I dug a lot of Indian head pennies there from the 1870's and up at less the 4 inches!
Regards, SkyPilot