Early miners learned that metal came from ore and ore often resided in narrow seams or reefs within the earth. To follow narrow seams of ore into mountains or underground required an improvement we call shaft mining. Rather than excavate a huge and largely worthless pit, the miner followed the seam with a narrow shaft, usually supported with timber to prevent cave-ins. Ancient shaft mines in Europe reached impressive depths. Perfectly preserved Celtic salt miners have been found in deep salt mines, where they lost their lives thousands of years ago.