It wasn't the guards, guard dogs, or barbwire fences at Stalag Luft III that were the biggest problem inmates faced: it was the dirt. On top was dusty grey, but not far underneath was sandy yellow. Any yellow dirt that turned up in the prison meant a tunnel was being dug. Tunnels, like the three used in the Great Escape were being dug all the time, but most of these were discovered because of the amount of time and yellow dirt required to dig from one of the prison buildings.
There had to be a way around it. Together, the three men built a really big pommel horse (the rail with a pair of handles, like gymnasts use), capable of holding up to three men uncomfortably inside. Then they convinced the guards that they, and many other inmates, just loved the hell out of gymnastics. To make it convincing, they practiced for hours each day, despite the fact their rations, while adequate, weren't exactly chalk full of protein.
The men took turns hiding inside the horse; inmates carried it in and out to the yard, placing it in the same spot by the fence every day (Closer to fence = less dirt). From inside, a digger took the top layer of grey dust and placed it in a box. Bowls were used for shovels. So as not to leave a gaping hole in the yard, a board was placed over the hole and covered with the grey dust from the box. Guards walked right over it, and didn't notice.
The yellow dirt, meanwhile, was brought inside the prison with the digger, where it was disposed of in gardens, rooftops, and the toilet, Shawshank-style. The noise from digging, which would be picked up by microphones placed along the fence line, was attributed to the gymnasts leaping around the yard.
Just me and my leotards, no digging going on here...