You are looking at an extract of a map concerning the Battle of Messines on June 7, 1917, which kicked off the 3rd Battle of Ypres and concluded with the capture of Passchendaele in late November 1917.
The area is a few miles south of Ypres and the total area inside the blue line is 25 square miles. There are 119,621 British and Allied soldiers hitting the German lines on the Messines Ridge. There were 19 large mines detonated by Engineer Tunneling Companies, 5 of these were dug in by Canadians – in the north above the map on the right you see two circles at the Hill 60 site dug by 3rd Canadian Tunneling Company under Major Angus Davies. A bit left of that you see the Queen Victoria Crater – dug by 1st Canadian Tunneling Company under Major Cy North. In the south are two more done by Major Davies. Major North was a mining engineer and expert at this kind of work. A YouTube Video shows what demolition of Ripple Rock near Campbell River on Vancouver Island looked like on April 5, 1958. The troops of the attacking forces might have seen one and at least two similar sized explosions. The effects on the German forces were catastrophic. They reported they suspected mining by the British but the bureaucracy discounted this. In the south two more mines were set off by Maj Davies. See a summary here of the events that led to the 19 mines being placed. We show a map of the Queen Victoria site here.
We will expand on this story as time permits.