These ports had, during the past two years, become more and more important to the U-boat campaign, and had therefore been more and more strongly guarded and fortified against attack. The section of coast upon which they lie had a system of defensive batteries, which included no less than 120 heavy guns, some of them of 15-inch calibre. A battery of these was upon the Mole at Zeebrugge--a solid stone breakwater more than a mile long, which contained also a railway terminus, a seaplane station, huge sheds for personnel and material, and, at the extreme seaward end, a lighthouse with searchlight and range-finder. An attacking force must reckon with a large number of defenders upon the Mole alone, besides the batteries and reinforcements on shore, and the destroyers and other ships in the harbour. But the attack on the Mole was an indispensable part of the enterprise; for the enemy’s attention must be diverted from the block-ships, which were to arrive during the fight and sink themselves in the mouth of the canal. And in order to deal satisfactorily with the Mole, it must be cut off from the reinforcements on shore by the destruction of the railway viaduct which formed the landward end of it. That was not all. The main difficulty of the plan was the management of the approach and return of the expedition. The conditions were extremely severe. First, the attacking force must effect a complete surprise and reach the Mole before the guns of the defence could be brought to bear upon them. The enemy searchlights must therefore be put out of action, as far as possible, by an artificial fog or smoke-screen; but again, this must not be dense enough to obscure the approach entirely. Secondly, the work must be done in very short time, and to the minute; for though the attack might be a surprise, the return voyage must be made under fire. The shore batteries were known to have a destructive range of sixteen miles; to clear out of the danger zone would take the flotilla two hours, and daylight would begin by 3.30 A.M. It was, therefore, necessary to leave the Mole by 1.30; and as, for similar reasons, it was impossible to arrive before midnight, an hour and a half was all that the time-table could allow for fighting, blocking, and getting away again. To do things as exactly as this, a night must be chosen when wind, weather and tide would all be favourable. We need not be surprised at hearing that the expedition had twice before started and been compelled to return without reaching its objective--once it was actually within fifteen miles of the Mole--but fortunately the Germans, having no efficient patrol at sea, got no hint of what was being planned; and in the end were so completely taken by surprise, that some of their guns when captured had not even had the covers removed from them!