Surface Mining vs. Underground Mining
There are basically two types of mining. Most mining in this area was underground. However, the open pit that became known as the #21 Mine in Mineville was an attempt to strip mine Adirondack magnetite.
Strip mining is a type of surface mining that involves removing the soil and rock lying over the mineral deposit (called overburden) that is being mined. Strip mining is for shallow, mostly horizontal deposits, usually extended over a considerable area.
Underground mines are large tunnels dug deep into the ground to extract minerals that are too far from the surface to be reached by surface mines—from veins that are more vertical. Miners quickly discovered that most of the iron deposits in the Adirondack region were at very steep, vertical angles. As a result, the deposits were only going to be effectively mined by the underground method, which required digging deep tunnels and shafts to access the iron deposits. Rich veins of iron still exist beneath Moriah. By the twentieth century, the deep vertical veins would prove uneconomical to mine, compared to the iron ore that could be strip mined from the surface in open pits in places like the Mesabi Range in Minnesota.