A depth bomb mortar and a set of "ash cans" at the stern of an American destroyer 257 A depth bomb mortar in action and a depth bomb snapped as it is being hurled through the air 260 Airplane stunning a U-boat with a depth bomb 261 The false hatch of a mystery ship 268 The same hatch opened to disclose the 3-inch gun and crew 268 A French hydrophone installation with which the presence of submarines was detected 269 Section of a captured mine-laying U-boat 272 A paravane hauled up with a shark caught in its jaws 273 A Dutch mine-sweeper engaged in clearing the North Sea of German mines 288 Hooking up enemy anchored mines 289 An Italian "sea tank" climbing over a harbor boom 300 Deck of a British aircraft mothership or "hush ship" 301 Electrically propelled boat or surface torpedo, attacking a warship 304 Hauling a seaplane up on a barge so that it may be towed 305 Climbing into an armored diving suit 320 Lowering an armored diver into the water 320 A diver's sea sled ready to be towed along the bed of the sea 321 The sea sled on land showing the forward horizontal and after vertical rudders 321 The diving sphere built for deep sea salvage operations 324 The pneumatic breakwater 325 INVENTIONS OF THE GREAT WAR CHAPTER I THE WAR IN AND UNDER THE GROUND For years the Germans had been preparing for war. The whole world knew this, but it had no idea how elaborate were their preparations, and how these were carried out to the very minutest detail. When the call to arms was sounded, it was a matter of only a few hours before a vast army had been assembled--fully armed, completely equipped, ready to swarm over the frontiers into Belgium and thence into France. It took much longer for the French to raise their armies of defense, and still longer for the British to furnish France with any adequate help. Despite the heroic resistance of Belgium, the Entente Allies were unprepared to stem the tide of German soldiers who poured into the northern part of France.