During the winter of 1914–1915, no serious bombardments took place before April. Both sides organized themselves on their positions, excavating shallow shelters which were braced with wooden beams and roofed with two or three layers of logs, over which earth was more or less thickly packed. Such shelters resisted well enough 150 mm. shells. But, in 1917, during the operations in Artois and Champagne, the adoption of larger calibres, and the use of torpedoes fired by trench machines, compelled the belligerents to bury themselves more deeply in the ground, wherever the soil permitted, and to build more solid shelters. When water interfered with deep excavations, the shelters were covered in with very strong T-shaped iron railway ties, or with several layers of steel rails; but these proved insufficient, and the Germans were the first to construct those bombproofs of reinforced concrete which the British for the first time encountered on the Somme. The concrete blocks are very large and the steel reinforcing bars extremely strong. Such works certainly impede and delay the operations of the enemy, especially when they are extensively employed, but events have proved that, given time, they can always be destroyed by gun-fire. The French have constructed similar works only at points of capital importance. They prefer the old wooden shelters, well reinforced with earth. [Illustration: Ailles and its western approach February 10, 1917--2.30 P.M.] At all the inhabited places in their lines, and at points of natural strength, the Germans have organized independent centres of resistance. They have transformed whole villages into fortresses. Mention need only be made of the labyrinth of Carency-Thiepval, Beaumont-Hamel, the tunnels of Cornillet, of Hill 304, of Mort-Homme, etc. In every one of those places a surprising number of concrete constructions and superimposed subterranean galleries were discovered. In them the enemy had collected reserve troops, food, and munitions. Such shelters, no doubt, afforded the Germans great protection, and in order to destroy them, it was necessary to have recourse to more and more powerful methods.