[2] Following consultations between the Engineer-in-Chief of the BEF, Brigadier George Fowke, and the mining specialist John Norton-Griffiths, the War Office formally approved the tunnelling company scheme on 19 February 1915.

[4] To make the tunnels safer and quicker to deploy, the British Army enlisted experienced coal miners, many outside their nominal recruitment policy.

These companies each comprised 5 officers and 269 sappers; they were aided by additional infantrymen who were temporarily attached to the tunnellers as required, which almost doubled their numbers.

[2] The success of the first tunnelling companies formed under Norton-Griffiths' command led to mining being made a separate branch of the Engineer-in-Chief's office under Major-General S.R.

[5] The formation of twelve new tunnelling companies, between July and October 1915, helped to bring more men into action in other parts of the Western Front.

In mid-March the Germans blew another large mine at Zwarteleen, creating a 9.1-metre (30 ft) deep crater and damaging their own lines in the process.

[10] Work beneath Hill 60 began early in March and three tunnels were begun towards the German line about 50 yards (46 m) away, a pit first having been dug some 16 feet (4.9 m) deep.

[1] The unit began sinking shafts and driving galleries to counter an enemy mining initiative immediately to the south and east of Loos.

[15] In spring of 1917, 173rd Tunnelling Company moved to the Ypres Canal sector near Boezinge[1] where it commenced work on the dugout at Yorkshire Trench.

The completed Yorkshire Trench dugout then served as headquarters for the 13th and 16th Battalions of the Royal Welch Fusiliers at the start of the Battle of Passchendaele later that year.

On 25 March 1918, personnel of the unit were converted into infantry – called No 2 RE Battalion – for emergency purposes, along with other Royal Engineers troops from XIX Corps (see 258th Tunnelling Company).

[1] In April 1918, the 173rd and several other tunnelling companies (171st, 183rd, 184th, 255th, 258th and 3rd Australian) were forced to move from their camps at Boeschepe, when the enemy broke through the Lys positions during the Spring Offensive.

[1] The operation to construct these fortifications between Reningelst and Saint-Omer was carried out jointly by the British 171st, 173rd, 183rd, 184th, 255th, 258th, 3rd Canadian and 3rd Australian Tunnelling Companies.