I saw it when it was discovered.
Had the 'bomb' gone off, very little would have happened immediately. The oak plug was thoroughly investigated and a hole drilled in it. The water level behind it was not up to the ceiling and in any case, the old miners, who had put in the 'plug' had backfilled the level behind it with rocks.
It was just after this that UU drilled down and filled the area with grout. All perfectly safe now.
I think, if it had gone off, 'whoever' set it hadn't much of an idea of explosives, as much of the force would have been dissipated and certainly the Coniston Mines stopes are so large under that area that the water would have taken ages to get out. Despite the Westmoreland Gazette I very much doubt if a tidal wave was ever on the cards.
The finest workers in stone are not copper or steel tools, but the gentle touches of air and water working at their leisure with a liberal allowance of time.
Henry David Thoreau