The fog-bank was nowhere less than a hundred feet thick--a cloud of impalpable particles impenetrable to any eye or any camera, however shrewdly filtered. And under that mattress of pale opacity the tanks crawled heavily. They lurched and rumbled upon their deadly errands, uncouth and barbarous, listening for each other by a myriad of devices, locked in desperate, short-range conflict when they came upon each other, and emitting clouds of deadly vapor, against which gas-masks were no protection, when they came upon opposing infantry. * * * * * The infantrymen, though, were few. Their principal purpose was the reporting of the approach or passage of tanks, and trenches were of no service to them. They occupied unarmed little listening-posts with field telephones, small wireless or ground buzzer sets for reporting the enemy before he overwhelmed them. They held small pill-boxes, fitted with anti-tank guns which sometimes--if rarely--managed to get home a shell, aimed largely by sound, before the tank rolled over gun and gunners alike. And now Sergeant Coffee and Corporal Wallis groped about in that blinding mist. There had been two systems of listening-posts hidden in it, each of admittedly little fighting value, but each one deep and composed of an infinity of little pin-point posts where two or three men were stationed. The American posts, by their reports, had assured the command that all enemy tanks were on the other side of a certain definite line. Their own tanks, receiving recognition signals, passed and repassed among them, prowling in quest of invaders. The enemy tanks crawled upon the same grisly patrol on their own side. But two miles of the American front had suddenly gone silent. A hundred telephones had ceased to make reports along the line nearest the enemy. As Coffee and Wallis stumbled about the little pill-box, looking for some inkling of the way in which the original occupants of the small strong-point had been wiped out, the second line of observation-posts began to go dead. Now one, now another abruptly ceased to communicate. Half a dozen were in actual conversation with their sector headquarters, and broke off between words. The wires remained intact. But in fifteen nerve-racking minutes a second hundred posts ceased to make reports and ceased to answer the inquiry-signal. G.H.Q. was demanding explanations in crisp accents that told the matter was being taken very seriously indeed. And then, as the officer in command of the second-line sector headquarters was explaining frenziedly that he was doing all any man could do, he stopped short between two words and thereafter he, also, ceased to communicate.