But Hamerton had little leisure to observe these things. The sergeant waited until he had received the password for the Sandinsel portion of the fortress, and then gave the order to march.
The enclosed way seemed interminable. For one thing, it never ran in a straight line for more than twenty yards at one time. Here and there other galleries branched both to left and right—the majority to the right. In places huge armoured bridges crossed the concealed way. On one a travelling crane, electrically propelled, was in the act of crossing.
The whole place seemed alive, judging by the sounds. The Sub could hear the sharp rattle of pneumatic riveters, the peculiar scroop of electric drills, and the thud of ponderous hammers, punctuated by the deafening hiss of compressed air. From a greater distance came the monotonous grinding sounds of a fleet of dredgers at work. There was no mistaking that: Hamerton had seen and heard dredgers at work in the principal naval ports at home. The groaning, rasping noise as the heavily-laden buckets come jerkily up the "ladder", the succession of dull thuds as bucket after bucket throws its contents of mud, sand, and larger stones down the shoot into the hopper, could not be mistaken for anything else.
Hamerton was puzzled. A fleet of dredgers—or even a solitary one—could be heard miles off. Yet during his enforced detention at Heligoland he had never heard the faintest sound that suggested operations of that nature.
"Well, it's either one of two things," he thought. "Either the cliffs of Heligoland possess strange acoustic properties, and deflect the sound entirely, or else these vessels have started work to-night for the first time—at least, since I set foot upon the island."
His musings were cut short by his escort coming to an abrupt halt outside a postern. Here, in front of the neutral-grey-coloured sentry-box—which had recently superseded the parti-coloured diagonal lines for which the German military authorities had a predilection—stood a sentinel with his bayonet at the "ready".
Even when the sergeant of the escort gave the password he was not permitted to pass the gateway until the rest of the corporal's guard had been turned out. Those responsible for the safety of the West Kalbertan Battery had evidently made up their minds to take no chances.
Hamerton saw very little of the interior of the fort that had only recently been constructed on foundations formed of ferro-concrete piles sunk twenty feet through the sand and another thirty feet in the stiff clay that composed the subsoil.
The West Kalbertan Battery had been well placed. Its armament, consisting of six fifteen-inch guns and a number of light quick-firers, commanded the only approach to North Haven from the north. It was one of a chain of six batteries—the others being the East Kalbertan, the Krid Brunnen, the Düne, the Aar de Brunnen, and the Sud Sandinsel—that were supposed to render Sandinsel impregnable. It was this heavy fortifying of the former sandbank that caused the German engineers to mount nearly all their heavy guns on Heligoland on the south-west side.
Then, although the guns of Heligoland commanded the approaches to the Elbe and the Weser, the batteries of Sandinsel were actually the key of the position. Whoever was master of Sandinsel was master of Heligoland.