In your article about engineering activities during the First World War (NCE 5 November) you describe the 19 mines which were laid under the Messines Ridge. When we visited the area we were told that 22 were actually laid. One was discovered by the Germans, 19 exploded on cue but two failed to explode.
One of these exploded much later when a tree which had grown over it was hit by lightning. One is still there!
John Haines (M),112 Keslake Road, London NW6 6DG.