THE THIRD BATTLE OF YPRES 1917 – PART FOUR Planning for a British offensive out of the Ypres Salient had been going on since 1915, but so far coalition politics and lack of resources had prevented it happening. Now, with the absolute urgency of taking the pressure off the French while their army was restored to its allegiance, the time had come. Before anything could happen in the Salient itself, Messines Ridge had to be dealt with. This was a ridge of land running south from the Salient which had been held by the Germans since 1914 and which dominated much of the Salient itself. In order to move the troops and pre-position the stores and artillery ammunition necessary to mount a breakout this dominance would have to be negated. General Plumer had long planned to do just that, with twenty-one enormous mines totalling one million pounds of explosive laid under the ridge by tunnelling companies of the Royal Engineers.
Interesting tidbit about the ash plants and a search quickly found some good images.