This topic might be a bit old already, but just yesterday I went on the trip to find my first iron ore myself, having had no "random luck" while caving so far. So I went just a bit south of my base with my trusty propick and followed a miniscule reading of limonite until I got it up to "very poor" with 1,6-1,8‰. This was the best I could get for the beginning as it didn't go any higher than very poor, so I wanted to try my luck and even found a little cave entrance. Inside the cave I found a shaft that goes very deep and that very shaft went right through an iron deposit! So I started digging
