Actually most shaft mines have to have pumps running continutously to keep water out of the mine.
If the mine had been inactive for an extended period you would have to expect that the mine have filled with water.
Like I said above they should have thoroughly understood the status of the mine before they started poking around.
Depends on the groundwater level in the surrounding rock. If there are mountains surrounding the shaft or adit, you’d have a pretty constant flow of water. In this case, the water level seems to have been above the ground level and dammed up.