I think, Roger, that it is very hard to imagine the original topography of the area in the early to mid 1700's now, as so much has changed. 'Upper' Worsley, ie: the land above the tunnel outlets, sits on a bluff of sorts, with the land rising steadily for about five miles towards Farnworth. Many early adits were driven from the original workings out to low lying areas, streams, and valleys in the local, and beyond, some of which were put to later uses.
Worsley itself sits on a limestone ridge, which ends shortly after the village, looking West. If you stand near to the A road leading towards the M60, and look out towards Boothstown, the land has dropped over 20' as far as the eye can see, mainly due to mine subsidence, and the draining of Chat Moss in the early part of the last century. Quite a panic ensued during the 1960's, because the main line of the canal was also sinking, along with the the surrounding land, and so the banks were raised using pit shale, brought by boat from Mossley Common pit. However, this of coarse raised the water level, depths of over 28' being recorded in places. It was then realised that the banks couldn't take that pressure of water, and so an emergency programme of filling the canal with watered down pit shale was started. Hundreds of thousands of tons of this was 'sprayed' onto the canal, between Leigh and Worsley, using adapted boats fitted with pumps and huge fire monitors, borrowed from Liverpool docks. (There is a photo somewhere depicting this, I will try and find it).
After some five years work, the canal was stabilised, and the land stopped dropping.
It is not known how many coal workings, or adits, were constructed in the area, however, a short walk along the canal towards Boothstown, past the crane and it's stop planks, brings one to a view of a submerged arch, now just the top of which is still showing. That at one time was another mine-canal outlet, before the land fell away. Opposite, and on the tow path side, there are the remains of the original canal line to Irlam, about half a mile was constructed before the project was abandoned. A strange twist to this is that the area was (is)? still known as Botany Bay, because it was dug by sentenced petty criminals awaiting transportation from Liverpool to Australia.
As I said earlier, the land to the East and North East rises towards Walkden and Farnworth. This of coarse makes the underground system much deeper as it progresses into the hillside. Above the basin, and seen from the path that leads into Worsley Woods, is the large reservoir which once fed all the industry that sprang up around The Dukes estates, including Brindley's 'Water Crane'. Now mostly silted up, it was quite deep at one period.
Of coarse your correct about the pumping. Steam pumps were employed as the seams grew deeper, however, such was the high quality of the coal being won, that the cost of installing pumps was fully justified. When Ashton Fields Pit closed in the seventies, (I think), pumping finally stopped, and there was some concern as the waters underground rose, that pollutants would be exited into the canal itself. The old boat entrances were bricked up, (as if that could stop the water), but then it was found that all the old, unused and unknown adits were back in their original use, some now under local houses, and so the brickwork was removed. As it was, no pollutants were found.
Another panic ensued when it was suggested that large quantities of coal gas, or methane, might be pushed up with the water, but the Worsley pits had always been pretty gas free, and so that silliness soon ended.
As a footnote to this, (please excuse the rambling), Fletchers Canal at Swinton, (the link from the Manchester Bolton and Bury Canal to Wet Earth Pit), there was a short section, (about half a mile), of canal tunnel built into the hill at Swinton. The idea was to link into The Dukes underground system above the inclined plain, but this project was also abandoned. During the construction of the M62 in the 70s, large amounts of material were removed from Wet Earth to form motorway bankings. This area has now filled in with water, and is known as Swinton Marinas. During the excavations, a short section of underground canal was discovered, along with two 'deep' boats, well preserved. I don't know what happened to them. Also, it is recorded that in 1912, a farmer was leading his horse and cart across a field near to Worsley, when the ground opened up a coal shaft, swallowing both the horse and the cart. Neither was recovered!