Prior to the Battle of Vimy Ridge, the British tunnelling companies also secretly laid 13 mines under German positions to destroy surface fortifications before the assault. To protect some advancing troops from German machine gun fire, as they crossed no man's land during the attack, eight smaller "Wombat" charges were laid at the end of the subways to allow troops to move more quickly and safely enter the German trench system by creating an elongated trench-depth crater that spanned the length of no man's land. At the same time, 19 crater groups existed along this section of the Western Front, each with several large craters. To assess the consequences of infantry having to advance across cratered ground after a mining attack, officers from the Canadian Corps visited La Boisselle and Fricourt where the mines had been blown on the First day of the Somme. Their reports and the experience of the Canadians at The Actions of St Eloi Craters in April 1916, where mines had so altered and damaged the landscape as to render occupation of the mine craters by the infantry all but impossible, led to the decision to remove offensive mining from the central sector allocated to the Canadian Corps at Vimy Ridge. Further British mines in the area were vetoed following the blowing by the Germans on 23 March 1917 of nine craters along no man's land as it was probable that the Germans were aiming to restrict an Allied attack to predictable points. The three mines already laid by 172nd Tunnelling Company were also dropped from the British plans. The mines were left in place after the assault and were only removed in the 1990s. Another mine, prepared by 176th Tunnelling Company against the German strongpoint known as the Pimple, was not completed in time for the attack. The gallery had been pushed silently through the clay, avoiding the sandy and chalky layers of the Vimy Ridge but by 9 April 1917 was still short of its target. In the end, two mines were blown before the attack, while three mines and two "Wombat" charges were fired to support the attack, including those forming a northern flank.