29 March 1943. A South Compound for Americans was opened in September 1943. USAAF prisoners began arriving at the camp in significant numbers the following month and the West Compound was opened in July 1944 for US officers. Each compound consisted of fifteen single-story huts. Each bunkroom slept fifteen men in five triple-deck bunks. Eventually the camp grew to approximately in size and housed about 2,500 Royal Air Force officers, about 7,500 US Army Air Forces, and about 900 officers from other Allied air forces, for a total of 10,949 inmates, including some support officers. The prison camp had a number of design features that made escape extremely difficult. The digging of escape tunnels, in particular, was made difficult by several factors: the barracks housing the prisoners were raised approximately off the ground to make it easier for guards to detect tunnelling; the camp had been constructed on land that had a very sandy subsoil; the surface soil was dark grey, so it could easily be detected if anyone dumped the brighter, yellow sand found beneath it above ground, or even just had some of it on their clothing. The loose, collapsible sand meant the structural integrity of any tunnel would be very poor. A third defence against tunnelling was the placement of seismograph microphones around the perimeter of the camp, which were expected to detect any sounds of digging. A substantial library with schooling facilities was available, where many POWs earned degrees such as languages, engineering or law. The exams were supplied by the Red Cross and supervised by academics such as a Master of King's College who was a POW in Luft III. The prisoners also built a theatre and put on high-quality bi-weekly performances featuring all the current West End shows. The prisoners used the camp amplifier to broadcast a news and music radio station they named Station KRGY, short for Kriegsgefangener (POWs) and also published two newspapers, the Circuit and the Kriegie Times, which were issued four times a week. POWs operated a system whereby newcomers to the camp were vetted, to prevent German agents from infiltrating their ranks. Any POW who could not be vouched for by two POWs who knew the prisoner by sight was severely interrogated and afterwards escorted continually by other prisoners, until such time as he was deemed to be a genuine Allied POW. Several infiltrators were discovered by this method