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 sand was bright yellow, so it could easily be detected if anyone dumped the darker, grey dirt 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.