Plumer's plan for Messines Ridge included a very ancient tactic to overcome the fortress of the enemy. He would tunnel beneath the lines and lay massive explosive charges beneath the key strong points to destroy them at the moment of the attack.
This tactic had been used on a smaller scale from early in the war, most famously at Hill 60, the much fought over high point close to Ypres, well known for the underground war as each side tried to out flank the other with silently dug tunnels, explosives and counter mines.
The mines at Messines Ridge were carefully planned, the geology studied in detail and small scale tests were carried out by the Royal Engineers in the UK and France. Tunneling for the Messines Offensive began in early 1915, no easy task in the dense blue clay of Flanders.
All the work had to be done in secret and in silence. Specialist Tunneling Companies were formed in the Royal Engineers, made up not of soldiers, but of miners, men who were used to working underground in the coal mines of the north of England, Canada and Australia and the men who worked on the tunnels for underground railways and canal systems. They began work behind the British Lines, with mine engineers sinking deep shafts through the layers of sand into the firmer clay below.
As the Second Army prepared in the training camps of Britain, including the New Zealand Rifle Brigade at Brocton Camp, the workers of the explosive factories prepared 600 tons of deadly ammonal, the miners extended the tunnels, inch by silent inch.
By the spring of 1917 Twenty Four mines had been laid, from twelve tunnels crossing No Man's Land 70 feet below the surface.