Directly the rounds had made their usual inspection and had taken their departure, Alec and his R.A.F. comrade set to work. With only a marline-spike and two pairs of hands, the task of removing the cemented-in stone was a tedious and formidable one. They had to proceed cautiously and silently, lest the alert sentries detected the grating of cold steel against hard cement. At intervals they desisted to listen. It was quite possible that the communication-tunnel might still be in use, in which case it was falling out of the frying-pan into the fire with a vengeance, and at the expense of a terrific amount of hard and purposeless toil. "Wonder how goes the time," gasped Smith, pausing to straighten his aching back and to wipe the perspiration from his forehead. "Getting on for midnight, I should say." "No fear; it's not much past eight o'clock," replied Alec. "I'll just see." He went to the seaward aperture and gazed skywards. The night was dark and calm. The stars shone brilliantly, although obscured here and there by patches of mist. In the northern sky the Great Bear flamed in stellar splendour. By its position with relation to the Pole Star, Alec was able to confirm his surmise with a fair degree of accuracy. "It's certainly not nine yet," he reported. "We've eight hours of darkness; ought to do something in that time. By Jove, this cement's hard! Wonder if it came from England?" He took the marline-spike from his companion. It was wet and sticky. The pilot's hands, hitherto well kept and unused to hard manual labour, were almost raw. Alec's were not much better, while every muscle in his body and limbs was aching with the unwonted exertion. Yet doggedly they continued their work, each man relieving the other at, roughly, a quarter of an hour's interval. The stone was beginning to show signs of working loose. "Wonder if any of our fellows will be over to-night," remarked the airman. "We don't give Fritz much rest." "It's been quieter to-day than ever since I've been here," said Alec. "You were the last fellow to come over." "And stay here," added the other grimly. "Hope Fritz doesn't think that one man being brought down will put the others off. If so, he's vastly mistaken." "I wish there would be a big raid or bombardment," declared the Sub. "We'd have to run the risk of being strafed; but, on the other hand, Fritz would be much too busy to worry about us. What's the weight of this stone: three-quarters of a hundredweight?" "Quite," replied Smith promptly. He had been mentally calculating the cubic capacity and weight of that wedge-shaped piece of stone for hours past. "It's not the weight that matters so much. It's the awkward shape of the brute." For the next ten minutes the two toilers were silent. Every jab with the now-blunted marline-spike was telling. The stone was almost ready for removal.