I remember using my Garrett DeepseekerADS III in the early 80's. I dug a balled up cigarette pack at elbow deep. That was 15" measuring tip of fingers to elbow.
I've not dug a target that deep since I returned with modern machines. That was about 34 years ago - and Garrett rocked!
I do wish I still had my ADS III, lost along the way in my travels.....