In spring 1917, Allied troops on the Western Front began the largest ever artillery barrage on German positions, using over 2.7 million shells. During the battle they succeeded in capturing the famous Vimy Ridge. But the ultimate cost of fighting was immense, with a daily casualty rate 40% greater than the Somme and almost double that of Passchendaele - making it hour for hour the most dangerous major campaign of the First World War.