Under such distressing conditions the U-boat had only a single chance of saving itself; when the water was sufficiently shallow--not deeper than three hundred feet--it could safely sink to the bottom and "play dead," hoping that the chasers, with their accursed listening devices, would tire of the vigil and return to port. A submarine, if in very good condition, could remain silently on the bottom for two or three days. The listeners on the chaser tubes presently heard sounds which suggested that their enemy was perhaps resorting to this manoeuvre. But there were other noises which indicated that possibly this sinking to the bottom was not voluntary. The listeners clearly heard a scraping and a straining as though the boat was making terrific attempts to rise. There was a lumbering noise, such as might be made by a heavy object trying to drag its hulk along the muddy bottom; this was followed by silence, showing that the wounded vessel could advance only a few yards. A terrible tragedy was clearly beginning down there in the slime of the ocean floor; a boat, with twenty-five or thirty human beings on board, was hopelessly caught, with nothing in sight except the most lingering death. The listeners on the chasers could follow events almost as clearly as though the inside of the U-boat could be seen; for every motion the vessel made, every effort that the crew put forth to rescue itself from this living hell, was registered on the delicate wires which reached the ears of the men on the surface. Suddenly sharp metallic sounds came up on the wires. They were clearly made by hammers beating on the steel body of the U-boat. "They are trying to make repairs," the listeners reported. If our subchasers had had any more depth charges, they would have promptly put these wretches out of their misery, but they had expended all their ammunition. Darkness was now closing in; our men saw that their vigil was to be a long one; they sent two chasers to Penzance, to get a new supply of bombs, and also sent a radio call for a destroyer. The spot where the submarine had bottomed was marked by a buoy; lanterns were hung out on this buoy; and two units of chasers, six boats in all, prepared to stand guard. At any moment, of course, the struggling U-boat might come to the surface, and it was necessary to have forces near by to fight or to accept surrender. All night long the chasers stood by; now and then the listeners reported scraping and straining noises from below, but these grew fainter and fainter, seeming almost to register the despair which must be seizing the hearts of the imprisoned Germans.