On occasion 1st Division Marines found evidence that the enemy had infiltrated their lines. It appeared the most likely spot for line-crossers to make their way into the Marine rear area was from the far bank of the Imjin between the Sachon and Han Rivers where the enemy MLR was only a short distance from the sector held by the 1st Amphibian Tractor Battalion. Two enemy agents “armed with pistols of German manufacture, six hand grenades, and one set of field glasses”[162] had been apprehended here by a Marine reconnaissance company patrol. The prisoners had stated they were “part of a force of one thousand men who were infiltrating to form a guerrilla force somewhere in South Korea.”[163] Six days later, after a brief fire fight between a small group of Chinese and a Marine outpost in the center of the division sector, the defenders discovered that two of the three enemy dead wore under their own clothing various articles of Marine uniforms. Neither of the Chinese had identification or any papers whatsoever. It was believed that both were enemy agents and that the attack on the outpost was a diversion “for the express purpose of detracting attention from infiltrators.”[164] [162] HqBn, 1stMarDiv ComdD, May 52, p. 27. [163] _Ibid._ [164] 1stMarDiv ComdD, Jun 52, p. 5. Even though enemy tactics and attempts to penetrate Marine positions demonstrated a good deal of soldierly skills, his conduct of defensive operations was nothing short of masterful. This was especially true of Chinese construction of underground earthworks. It appeared that the Chinese had no single pattern for this type of field fortification. Like the Japanese in World War II, the Chinese Communists were experts in organizing the ground thoroughly and in utilizing a seemingly inexhaustible supply of manpower to hollow out tunnels, air-raid shelters, living quarters, storage spaces, and mess halls. Americans described the Chinese as industrious diggers,[165] who excavated quickly and deeply for protection against UN bombardments. From numerous reports of ground clashes in the 1st Marine Division sector and from observations made by Marine pilots, it became known that the enemy was quick to seek cover whenever he was exposed to sustained artillery bombardment or air attack. [165] “The Chinese attack by ‘shovel’ proved effective and difficult to combat. They burrowed forward almost continuously, even under direct observation. Every foot of advance provided added opportunity to attack Marine COPs with greater impunity. While this activity possibly provided Marines with target practice in both small arms and mortars, these CCF working parties in a narrow trench 7 to 10-feet deep probably took very few casualties.” Col William R. Watson, Jr. ltr to Hd, HistBr, G-3 Div, HQMC, dtd 18 July 67.