 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.