There was a chalk formation here in which were many large natural caves. One enormous cave was said to have held thirty thousand soldiers, and in this section the Germans kept large reserve forces. By digging far into the ground, the German troops secured protection from shell-fire; in fact, the horrible noise of battle was heard only as a murmur, down in these depths. With characteristic thoroughness, the Germans built their trench system for a long stay; while the Allies, on the other hand, looked upon _their_ trenches as merely temporary quarters, which would hold the enemy at bay until they could build up armies large enough to drive the invaders out of the country. The construction of the trenches along some parts of the battle-line was particularly difficult, because of the problem of drainage. This was especially true in Flanders, where the trenches in many cases were below water-level, and elaborate pumping-systems had to be installed to keep them dry. Some of them were concrete-lined to make them waterproof. In the early stages of the war, before the trenches were drained, the men had to stand in water for a good part of the time, and the only way they could get about at all in the miry trenches was by having "duck-boards" in them. Duck-boards are sections of wooden sidewalk such as we find in small villages in this country, consisting of a couple of rails on which crosspieces of wood are nailed. These duck-boards fairly floated in the mud. [Illustration: Courtesy of "Scientific American" French Sappers using Stethoscopes to detect the Mining Operations of the Enemy] Some of the trenches were provided with barbed wire barriers or gates calculated to halt a raiding-party if it succeeded in getting into the trench. These gates were swung up out of the way, but when lowered they were kept closed with a rather complicated system of bolts which the enemy would be unable to unfasten without some delay; and while he was struggling to get through the gate, he would be a target for the bullets of the defenders. HIDING RAILROADS IN DITCHES