I can only recommend a visit to the "Ruhrpott" area of Germany. Probably <i>thousands</i> of mines were dug over the centuries, hundreds alone after WW2 when people dug for coal on their own under horrendous conditions, and none of them documented. Accidents and incidents aboveground happen frequently when old shafts collapse. A lot of former mining sites have been converted to museums, although none of them actually allow access at the old depth. You can spend a month in NRW and not be able to visit all the museum sites!<p>The entire Ruhrpott settled and sank so much that if the water pumps in the largest mines would cease operating for too long, the entire area would flood. It's literally called "Ewigkeitslasten" (forever burdens) for that reason.