When I got my 15" WOT (3kHz) last month, I tested it out in my yard the first evening. One particular place, I kept getting a solid, if narrow, 42-44 signal. However, no matter how I dug, I didn't find anything.
I spent this afternoon winterizing the garden and lawn and decided to give it a go with the shovel at that spot. After much sweat, it turned out to be a large, bright-green copper canning lid flat on the bottom of the hole...pretty dang near to 30 inches down. My detector probably thought it was the mother of all pennies; yes, these coils go deep (with the right target!)
I'm wondering how the heck the lid sank down that far, as I was completely through the top layer of soil and well into the heavy clay we often find around here.