What little I have researched says you have to pay a lot more to get beyond a few inches. However, the size and density of the object will alter the depth that any unit can go. Also, with earphones, you can pick up signals deeper than regular audio.
Admission of guilt here. I made a crack above about finding Spanish Gold but there is a range not far from here where the Spaniards mined a lot of gold over a 300 year period. On numerous occasions, the miners would eventually upset the natives and get themselves killed with their gold buried or thrown into the lakes. Quite a bit of both gold and silver ingots have been found as well as bags of high grade ore. So yes, one of the son-in-laws and I have been considering "goin diggin".:tinysmile_twink_t2: