Coal mining has been happening in this country since the roman times. The earlier the coal mining the closer to the surface it tends to be. So early miners would dig a hole down to the coal 1-4m and then dig around this hole until the sides started to collapse, then move across a couple of meters and start again. This is called bell pitting. As mining technology improved over time they got deeper and deeper.
Modern mining can be 1km underground and deeper.