Suddenly, a mineshaft (-1862,93,-937) is a very deep hole found by drinkmorecoffee.
A simple near-vertical shaft that would be very easy to stumble into. This screenshot was taken at the mouth, at the top of the hill, level 93. You can get down to about 62 and still look up to see this opening. The shaft continues downward until bottoming out at level 34. Iron and coal deposits are visible on the cave walls.