why would a metal detector go "bat crazy" in a flooded mine pit reservoir, verses any other sort of lake? Are you asking this because you're wondering if the material they mined (prior to making a lake out of the pit) might have some sort of mineral or metal that throws off the machine? If so, I guess it would depend on what they mined from the location. But off the top of my head, there should be no mineral (even if they mined for gold, for instance) that would stop a detector from working in any soil anywhere (both under the water, and above ground). Oh sure, some soils are worse than others (more mineralized), but barring jet black sand, you can usually find a detector to ground balance (if not automatically too for a lot of today's machines) to just about anywhere. The fact of it being underwater now, has no bearing on whether or not the machine works. I mean, you can simply try the shores of the lake too, and that's the soil composition of what's immediately adjacent, underwater. The adding of the fresh water isn't going to change that.
Even if they mined some type of metal (copper, gold, silver, etc...) from the location, you have to remember that it's trace particles that are processed out of tons and tons of soil. Ie.: there's no "copper nuggets" or "gold nuggets", etc... there. It's in powder form in trace amounts per the tons of soil, that is processed. So even soils that produce metals, do not preclude someone from metal detecting those soils. It simply means they're going to be a slightly mineralized soil.
And as far as what you're looking for, has this become a modern swimming lake? I mean, what are you going to be looking for, underwater, to begin with?
