The Y Sap mine was an underground explosive charge, secretly planted by the British during the First World War and ready for 1 July 1916, the first day on the Somme. 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. The mine was named after Y Sap, the British trench from which the gallery was driven. It was one of 19 mines on the British sector to be blown at the start of the battle.