HI franklyn: Ya don't suppose there was nothing deeper where you looked? snicker.
Nope, the tf 900 will go down 20 + ft on a refrig sized object. However, whatever you get, be sure that you get a ground neutralizing one.
The main reason for a two box is that you can cover much more ground with it in a given period of time , say 6 ft wide sweeps / passes, and they do not have to almost be in contact with the ground which can be a problem with grass, weeds, or brush and a standard coil..
Frankly, I would need a fantastic story to dig even 6 ft. So far I have never seen anything deliberately buried more than 4 - 5 ft, especially in the case of a single man burying something.
For serious ''treasure hunting', they cannot be beat. For coins and smaller objects, they are almost useless
Don Jose de La Mancha