History of Wigan   (Help with ancient and recent history.)

Peter,
The mining did result in subsidence/collapse... hence the 'Flashes' - large bodies of water where land had sunk. Even the railway track disappeared in Pennington, Leigh. Moving on, some canal locks have twisted due to mining subsidence.
Ena,
There is an old OS map that indicates Roman mining north of the town, but I guess it would be surface or bell pit rather than deep pits.