by daemmon » Sat Feb 19, 2011 8:35 pm
Actually, did you get any "There appears to be XXXX below" message? I'd suggest you to keep prospecting till you get that message.
Also, what worked for me, was to prospect in the cave, got the "There appears to be Iron Ore below", then I moved a little bit south from there, build the minehole and start mining north, so I know exactly where to go from there.
Also, if you want to prospect the cave remember at least 1 or 2 turns and "follow" the cave from above. I found minerals that way too. (Yes, my cave for some reason has Copper and Tin on layer 1, and Iron Ore on layer 2)