The digging of escape tunnels, in particular, was made difficult by several factors: the barracks housing the prisoners were raised approximately 60 cms 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.