Such were the subchaser tactics in their perfection; yet it was only after much experience that the procedure began to work with clock-like regularity. At first the new world under the water proved confusing to the listeners at the tubes. This watery domain was something entirely new in human experience. When Dr. Alexander Bell invented his first telephone an attempt was made to establish a complete circuit by using the earth itself; the result was that a conglomerate of noises--moanings, shriekings, howlings, and humming sounds--came over the wire, which seemed to have become the playground of a million devils. These were the noises, hitherto unknown, which are constantly being given out by Mother Earth herself. And now it was discovered that the under-ocean, which we usually think of as a silent place, is in reality extremely vocal. The listeners at the C- and K-tubes heard many sounds in addition to the ones which they were seeking. On the K-tubes a submarine running at full speed was audible from fifteen to twenty miles, but louder noises could be heard much farther away. The day might be bright, the water quiet, and there might not be a ship anywhere within the circle of the horizon, but suddenly the listener at the tube would hear a terrific explosion, and he would know that a torpedo, perhaps forty or fifty miles distant, had blown up a merchantman, or that some merchantman had struck a mine. Again he would catch the unmistakable "chug! chug! chug!" which he learned to identify as indicating the industrious and slow progress of a convoy of twenty or thirty ships. Then a rapid humming noise would come along the wire; that was the whirling propeller of a destroyer. A faint moan caused some bewilderment at first; but it was ultimately learned that this came from a wreck, lying at the bottom, and tossed from side to side by the current; it sounded like the sigh of a ghost, and the frequency with which it was heard told how densely the floor of the ocean was covered with victims of the submarines. The larger animal life of the sea also registered itself upon the tubes. Our listeners, after a little training, could identify a whale as soon as the peculiar noise it made in swimming reached the receivers. At first a school of porpoises increased their perplexities. The "swish! swish!" which marked their progress so closely resembled the noise of a submarine that it used to lead our men astray. But practice in this game was everything; after a few trips the listener easily distinguished between the porpoise and the submarine, though the distinction was so fine that he had difficulty in telling just how he made it. In fact, our men became so expert that, out of the miscellaneous noises which overwhelmed their ears whenever the tubes were dropped into the water, they were able almost invariably to select that of the U-boat.