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960 position. In July 1916 they blew four mines behind the British trenches, making a crater 150 yd. by 50 yd. The British infantry had been warned by the mine officers; there were few casualties, and a barrage having been arranged for, the Germans could not secure the position. Just before this the British had started a deep gallery about 100 ft. below the surface. Working from this, and after some fighting in shallow systems, they blew four charges, totalling about 10,000 Ib.

" Ten minutes before the blow two officers and eight men of a tunnelling company lay out in a shallow hole in a small crater which had been calculated as directly above the enemy workings. They started to dig down immediately after the blow, and broke through to the enemy gallery by midnight, which they found tamped with sandbags. These were all removed by 10 A.M. next morning, and about mid-day the galleries were entered by men wearing Proto apparatus a complete survey was made, and 700 ft. of undestroyed enemy gallery, together with much apparatus, was found to have been captured. The enemy system was connected to our own shallow workings, and for the first time in its history The Bluff was reported as absolutely safe from underground attack.

"This final operation was a good example of a successful flank attack; our gallery was well below the enemy, and we succeeded in getting under him and along his front without being discovered."

The Somme offensive in 1916 was an example of mining con- ceived and executed in connexion with an important attack, the plans being worked out in April, and a programme laid down. The underground work for this attack included mines for de- struction of enemy works, galleries to provide advanced emplace- ments for machine-guns and Stokes' mortars, to be developed into communication galleries, and galleries for emplacements, ammunition, etc., only. There were nine mines containing altogether nearly 200,000 Ib. of high explosive. They gave excel- lent results. The largest, having 40,600 Ib. of ammonal, was placed at a depth of 75 ft. at the end of a gallery 300 yd. long; and it destroyed a salient covering too yd. of German line. The special point about the " galleries to provide advanced emplace- ments " was that a tunnelling officer was stationed at the end of each gallery, whose duty it was to break through the top of the emplacements for mortars and machine-guns, and also to break out at the end of the gallery to start the communication trench. Of the galleries for communication purposes only, 16 were con- nected up with the German lines. The methods of communica- tion were three: by stepping up to the ground level and breaking out and digging; by placing a charge in a bore hole and blowing a crater in the enemy's trenches, then breaking out from the gal- lery into the crater, and thence into the German trenches; and by making a trench with a " pipe-pusher." Galleries for emplace- ments to ammunition depots were successfully broken through at the top, and the galleries were used at first for signal cables, runners and ammunition carriers; later for transport of wounded. This operation proved the value of these underground galleries in connexion with an attack.

In Sept. 1916, on the German side, large numbers of miners were sent home for industrial reasons, and mining activity was much reduced. The deep system controlled by G.H.Q., and di- rected against distant objects, was steadily pushed on, but the old shallow defensive systems were merely kept free of water and watched by listeners. The tunnellers thus set free were employed on what was practically a new development of mining, namely, construction of subways for infantry, and deep dug-outs.

By the end of Feb. 1917 nearly 20 m. of subways had been completed on the I., II. and III. Armies' fronts. In Sept. 1917 the Hohenzollern-Auchy subway system was completed, which made it possible to patrol 4 m. underground. At Nieuport sub- ways and dug-outs were successfully made in spite of great diffi- culties with water and running sand. At the German attack a subway, not quite completed, east of the Yser, saved a consider- able part of the infantry garrison.

The experience gained at the Somme was used to great advan- tage in preparing the attack of April 9 1917 on the Vimy ridge. In this case the mines, which were actually finished six months before the attack, were not of great importance, only two out of eight being used. The subways, which were made in three and a half months, were an interesting feature:

" Twelve infantry subways from reserve line to assaulting trenches. These subways averaged nalf-a-mile long each, the shortest being 290 yd. and the longest 1,880 yd. They were 6 ft. 6 in. high and 3 ft. wide, with at least 20 ft. of head-cover, and were lit with electric light throughout, Email lighting plants being installed in each subway.

"The subways were supplied with dug-outs, assembly chambers, trench-mortar and bomb stores, trench-mortar emplacements, water- tanks, dressing stations, signal offices, and in some cases brigade and battalion headquarters. They had numerous entrances and exits; the latter were broken out into advanced assaulting trenches on the last night, and the troops were able to file out into their assaulting positions through them. Maps on boards were hung at various points to show the position in relation to the surface. Tramways were laid in some of the subways, and these were found most useful ' for carrying up trench-mortar ammunition, stores and rations; signal cables were carried through the subways and signal stations were installed in them. The dressing stations, being dry and well lit, enabled the wounded to be easily attended to and kept in safety until they could be evacuated. Water-mains were also laid in some.

"Finally, the troops housed in the subways were able to rest in a safe, warm and dry place up to the time of the attack.

"The subways proved most successful, and, throughout, proof against bombardment, the only damage being done to some of the entrances. These were easily and quickly repaired by tunnelling company repair gangs, which were kept in each subway.

"The electric light in the subways was installed and run by the Australian electrical and mechanical mining and boring company. The lights were kept running throughout the whole operation without a breakdown. One plant actually ran 153 hours continuously. Ventilation was good, the only difficulty being in the case of gas attacks, when plenty of ventilation actually increased the danger. Specially appointed traffic officers (not tunnellers) were told off to each subway, and carefully trained beforehand in their duties."

In this connexion the use made of the Arras caves is interest- ing. These were underground quarries of the i7th century, and were discovered by chance. They were utilized as shelters for men by connecting them with the front-line subways. The main galleries were 6 ft: 6 in. high and 4 ft. wide. The caves and galleries were lit electrically throughout, and water-mains laid. The caves accommodated about 11,000 men.

At Messines (June 7 1917) the main feature was the enormous scale of the explosions, unprecedented in military history. The total charges were nearly 1,000,000 pounds. There were 20 mines distributed over a length of about 8 m., the greatest concentration being 500,000 Ib. on 4,500 yd. of front, from Hollandscheschuur to Ontario Farm. The largest mine was 95,600 Ib. of ammonal, at a depth of 125 ft., at St. Eloi. The longest gallery was 720 yards.

The first idea of a deep offensive in the Messines- Wytschaete area was in the summer of 1915, when a deep gallery was started against Hill 60. Various deep galleries were started in 1916, but the final scheme had not begun to take shape until the summer. Many mines were laid months before the explosion. From June 1916 to May 1917 the Germans were searching for mine systems and blowing deep camouflets with heavy charges. Galleries were damaged or flooded in many cases, but by strenuous work most of them were restored in time for the attack. A very large amount of dug-out accommodation was provided.

The use of these enormous mines, whose destructive effects could not be definitely foretold, was naturally a matter of anxiety, but they were certainly successful. Ludendorff writes in his memoirs:

"Weshould have succeeded in retaining the position but for the exceptionally powerful mines used by the British, which paved the way for their attack ....

"All had been quiet and no sound of underground work on the part of the enemy could be heard at our listening posts. The mines must therefore have been in position long before.

"The moral effect of the explosions was simply staggering; at several points our troops fell back before the onslaught of the enemy's infantry."

This was the last great operation with explosive mines. After this the tunnelling companies were largely employed upon dug- outs and tunnelling. For instance, they made a number of sub- ways through the canal bank near Boesinghe, for storage of material for bridging the canal. Subsequently they made ap- proaches to the bridges by blowing gaps in the bank with bore holes. Other tunnellers were employed on roads and tramways.

During the German offensive in 1918 the tunnellers were used