About a year later, the battle of Messines (https://en.m.wikipedia.org/wiki/Mines_in_the_Battle_of_Messi... ) saw larger mines, some of which with over 40,000 kg of explosives.
Five of them weren't detonated. One of those exploded in 1955 after a lightning strike. Most likely, four others are still lying below ground. It's anybody's guess whether they still are dangerous and might detonate one day.
"In late 1915 / early 1916, Commonwealth forces began developing a new form of barrage. Beginning close to their own lines, the 'creeping' barrage moved slowly forward, throwing up dirt clouds to obscure the infantry who advanced close behind. The barrage would reach the enemy lines and suppress as normal (by driving men into bunkers or more distant areas) but the attacking infantry would be close enough to storm these lines (once the barrage had crept further forward) before the enemy reacted. That was, at least, the theory."
From http://europeanhistory.about.com/od/worldwar1/p/prcreepingb....
Injuries are expensive to a military power.
"During The Somme, the British bombardment consisted of 1,537 guns firing over 1,500,000 shells over a period of 168 hours averaging 8,929 shells fired per minute."
That is just... staggering. I can't find a source for it, but I remember some factoid stating that on average it took several hundred shells to kill one soldier. That's not to say they had to take several hundred hits, but that on average that's how many shells it took to rack up one kill.
(from: http://www.tommy1418.com/wwi-facts--figures--myths.html)
*Edit for caps only