Hill 60 was valuable to the Germans because it overlooked the lower ground on which the British had dug their trenches. Observers on the hill could watch what was going on down below, and direct the fire of their heavy guns which were stationed a couple of miles or so to the rear. The whole hill was seamed with trenches and saps. The Germans held the upper slopes and the summit, and their positions were only fifty yards away from those of the British. They had strongly fortified the hill, because they knew that its loss would force them to give up a large part of their line. For this reason, and because it would afford us a gun position commanding much of the German front, we now prepared a bold attack upon it. Since the Battle of Neuve Chapelle and the counter-attack at St. Eloi there had been a lull in the fighting. Of course, the big guns on both sides fired daily, and the aeroplanes of friend and foe made constant raids and observation flights; but the infantry had been but little engaged. On 1st April an Allied aviator played a practical joke on the Germans. He flew over Lille, and dropped a football on the aerodrome.[34] It bounded up to a great height, and the Germans, thinking that it was a new kind of bomb, at once scuttled away into cover. The supposed bomb did not explode, and after a time they ventured out to examine it, and discovered this inscription on it: "April fool--Gott strafe England,"[35] In the early days of April our engineers, all unknown to the enemy, were busy driving galleries under Hill 60, and preparing mines. At seven o'clock on the morning of 17th April, when the 1st Royal West Kents and the 2nd King's Own Scottish Borderers were waiting with fixed bayonets, the mines under the hillock were exploded. There was a terrific roar, and it seemed as though the German trenches had been struck by an earthquake. Parapets, sand-bags, wire entanglements, and the bodies of the men were blown high into the air. The trenches disappeared, and in place of them yawned huge craters and mounds of piled earth. Before the dense columns of dust and smoke could subside, our guns belched forth shrapnel and high-explosive shells, so as to prevent the enemy from sending up reinforcements. In the midst of the whirlwind of shot and flame the Germans who had survived the explosions were seen falling over one another in their efforts to escape by means of the communication trenches. They were so panic-stricken that some of them forced a way to safety by charging through their own ranks with the bayonet.