The mine was dug by the Tunnelling Companies of the Royal Engineers under a German machine-gun nest known as Blinddarm (appendix) in the front line, on the north side of the village of La Boisselle in the Somme département.

A German advance westwards towards Albert was stopped by the French at La Boisselle and attempts to resume mobile warfare in October failed.

[1] Fighting continued in no man's land at the west end of La Boisselle, where the opposing lines were 200 yd (180 m) apart, even during lulls along the rest of the Somme front.

[2] At the end of July 1915, fresh troops were observed moving into the French positions north of the Somme and on 1 August, they were identified at Thiepval Wood as British soldiers ("dressed in brown suits").

On 24 July, the 174th Tunnelling Company established headquarters at Bray, taking over some 66 shafts at Carnoy, Fricourt, Maricourt and La Boisselle.

[4] No man's land just south-west of La Boisselle was very narrow, at one point about 50 yd (46 m) wide, and had become pockmarked by many chalk craters.

[8] The tunnelling companies were to help the Allied preparations for the battle by placing 19 mines of various sizes under the German positions along the front line and by preparing a series of shallow Russian saps from the British front line into no man's land, which would be opened at Zero Hour to allow the infantry to attack the German positions from a much shorter distance.

Flints were carefully prised out of the chalk and laid on the floor; if the bayonet was manipulated two-handed, an assistant caught the dislodged material.

Spoil was placed in sandbags, passed hand-to-hand along a row of miners sitting on the floor and stored along the side of the tunnel, later to be used to tamp the charge.

[14] Lochnagar and Y Sap mines were overcharged to ensure that large rims were formed from the disturbed ground and communication tunnels were also dug for use immediately after the first attack.

[18] The explosion of Y Sap mine left a large crater but failed to assist the British attack at Zero Hour as the Germans had detected the mine in time, evacuated the position and caught the British infantry in crossfire, particularly the troops of the 102nd (Tyneside Scottish) Brigade (Brigadier-General Trevor Tiernan), on the left flank of the 34th Division.

There it hung, or seemed to hang, for a moment in the air, like a silhouette of some great cypress tree, then fell away in a widening cone of dust and debris.

The barrage had lifted to the second-line trenches, the infantry were over the top, the attack had begun.The detonation of Y Sap and Lochnagar mines failed to neutralise the German defences in La Boisselle.

The mines were expected to provide some protection against German machine-gun fire, by creating mounds around the crater rims and a smoke screen was to cover La Boisselle at zero hour but the wind blew it away from the village.

Prior and Wilson criticised the bombardment plan for lifting the heavy artillery off the German front line at 7:00 a.m., thirty minutes before the infantry advance, which meant that its fire for the rest of the day was ineffective.