About the same time American units appeared on the Aisne fighting front in the vicinity of the famous Chemin-des-Dames. They had been detailed there for training purposes; but their location was not revealed by General Pershing until the Germans themselves knew of their presence. They had been there weeks before their presence became known. Suddenly ordered from their billets, they entrained to the railhead, and passed through mile after mile of shell-scarred, desolate ground and through several great piles of stones and débris which once were villages but now had not a single house standing. They took up their positions without a hitch to the music of roaring guns, both friendly and hostile, their flashes stabbing the blackness of the night, first here and then there, as far as the eye could see. Many of them were quartered in quarries twenty feet underground, one quarry having room for sheltering 3,000 men. Other recesses beneath the surface occupied by them were partly natural in formation and improved through blasting operations by the Germans, who occupied them for three years. But most of the American troops were above ground, having established themselves in trenches and dugouts which they had cleaned, strengthened, and improved and protected by barbed-wire entanglements. The cave dwellers' chief barracks was seventy-two feet underground. This cave ran in long galleries with cement ceilings, and was lighted by electricity, acetylene lamps, and oil lanterns. The troops slept on low, double-tiered wooden bunks provided with straw. Here the troops usually remained from four to six days each, a company being assigned to a certain portion of the cavern. Being new to active field service, they were not permitted to roam about at will, lest they be lost, nor to leave the cavern at all except on duty, lest they be detected by enemy airmen. The appearance of Americans on this sector was greeted by the Germans with a sign reading: "Welcome, Yankees." It was promptly riddled with bullets till it looked like a sieve. The French and Americans responded to this little pleasantry by an attack on the German lines at Chevregny on February 23, 1918. The French organized a little raid on the German lines, aided by twenty-six picked Americans, and rehearsed the operation the day before. An hour's barrage at dawn brought the Americans moving forward eagerly with their French comrades. They moved so fast, indeed, that they came within thirty yards of the dropping shells on reaching the enemy lines. It was the first time American troops had essayed an attack under such a curtain of fire, and their ardor actually took them beyond their objectives. In the German trenches officers were making the rounds after morning relief when the Americans and French burst in. The Germans rushed to shelter in a dugout roofed with rails and sandbags; but this refuge was immediately shattered by a French shell.