 At 2 : 50 a.m. on 7 June , the British artillery bombardment ceased ; expecting an immediate infantry assault , the German defenders returned to their forward positions . At 3 : 10 a.m. the mines were detonated , killing c . 10 @,@ 000 German soldiers and destroying most of the middle breastwork Ib of the front system , paralysing the survivors of the eleven German battalions in the front line , who were swiftly overrun . The explosions occurred while some of the German front line troops were being relieved , catching both groups in the blasts and British artillery fire resumed at the same moment as the explosions . Some of the <unk> ( <unk> ) in breastwork Ic were able to counter @-@ attack but were overwhelmed quickly , as the British advanced on the Sonne line , which usually held half of the support battalions but had been reduced to about 100 men and six machine @-@ guns , in each 800 yards ( 730 m ) regimental zone . Smoke and dust from the British barrage limited visibility to 100 yards ( 91 m ) and some defenders thought that figures moving towards them were German soldiers , retreating <unk> and were also overrun . After a pause , the British continued to the Höhen line , held by half of the support battalions , a company of each reserve battalion and 10 – 12 machine @-@ guns per regimental sector . Despite daylight , German defenders only saw occasional shapes in the dust and smoke , as they were deluged by artillery fire and machine @-@ gunned by swarms of British aircraft . The German defence in the south collapsed and uncovered the left flank of each unit further north in turn , forcing them to retire to the Sehnen ( Oosttaverne ) line . Some German units held out in Wijtschate and near St. Eloi , waiting to be relieved by counter @-@ attacks which never came . The garrison of the <unk> ( Caterpillar or spoil heap to the British ) held on for 36 hours until relieved . 
