During the siege, British and Spanish forces faced each other across an approximately 1 kilometre (0.62 mi) wide stretch of the marshy open ground that forms the isthmus immediately to the north of the Rock of Gibraltar.

There was no possibility of building a path there due to the vertical cliff face, so Sergeant-Major Henry Ince of the Military Artificers suggested digging a tunnel to reach it.

Colonel John Drinkwater Bethune, who wrote an account of the siege in 1785, described how this came about: [T]he mine was loaded with an unusual quantity of powder, and the explosion was so amazingly loud, that almost the whole of the Enemy's camp turned out at the report: but what must their surprise be, when they observed where the smoke issued!

– The original intention of this opening, was to communicate air to the workmen, who before were almost suffocated with the smoke which remained after blowing the different mines; but, on examining the aperture more closely, an idea was conceived of mounting a gun to bear on all the Enemy's batteries, excepting Fort Barbara: accordingly orders were given to enlarge the inner part [of the tunnel] for the recoil; and when finished, a twenty-four-pounder was mounted.

A consistent direction was eventually found, and by the end of the fourth siege embrasures had been blasted overlooking the Spanish lines.

[3] As an additional safety measure, each cannon was isolated with a wet cloth hanging above it from a rope, to prevent the sparks from igniting the remaining gunpowder.

[6] The Second World War led to another great wave of tunnelling as work was undertaken to enable the Rock to house a garrison of 16,000 men with water, food, ammunition and fuel supplies sufficient to last a year under siege.

[8] The Great Siege Tunnels were reused during the war; although it is uncertain exactly how they were used, it appears that they may have housed one of the generators used to power Gibraltar's searchlights, as a concrete mounting pad of the requisite dimensions was installed in one of the embrasures.