There is no definitive answer to this question as it depends on a variety of factors, including the type of metal detector being used, the composition of the soil, and the thickness of the object being detected. However, most handheld metal detectors can detect objects up to 8-10 inches below the surface, while some professional models can detect objects up to 20 feet below the surface. It is important to note that metal detectors are not able to detect objects that are buried deep in the ground, as the electromagnetic field cannot penetrate beyond a certain depth. Therefore, if you are looking for something that is buried deep underground, a metal detector is not likely to be of much use.