The largest mining operations were conducted in the Battle of Messines, where specialist Royal Engineer tunnelling companies placed 22 mines under German lines. 19 were eventually exploded, killing about 10,000 German soldiers. Underground attacks especially broke the morale of the enemy if he was surprised in his secure positions. However, the huge craters that are the results of mining enemy positions could be obstacles during an advance, as was found at Petersburg Battle of the Crater and the Somme. Tunnels built to secure frontline supply were built in several places on the Western Front.