mineshafts

The starting points are predetermined by your map seed. In average, every 100th chunk has a starting point for a mineshaft.
The height level of the room is less than y=53 (random).

Around the origin point of the map, the chances for abandoned mineshafts are actually smaller. The chunk with the coordinates (0,0) has a 0% chance to contain a starting point. The chance increases linearly in every direction until it is at 1% as soon as either chunk coordinate reaches an absolute value of 80 (1280 in standard coordinates).