Just as a dummy key to open the basement corridor door had been completed, somebody had a brain-wave which enabled the whole idea of using the cellar passage at all to be dispensed with. It was conjectured (correctly, as it turned out) that behind these planks there must be some sort of square cellar or chamber not actually in use by the Germans. Two sides of it would be bounded directly by the eastern and southern walls of the Kaserne, the western side by the last cellar in the basement corridor (the potato cellar) and the northern side by the inside wall of the corridor itself. If this supposition was correct, and if the place could be got at, it would be an ideal spot both as a base of operations for the tunnel and a receptacle for the excavated earth. It was decided therefore, by loosening one or more of the planks and hingeing them so that they could be moved as required in and out of position, to arrange a makeshift but effective trap-door for the daily needs of the working-party. The ceremony at the laying of the foundation stone—one should say, perhaps, removing the foundation plank—was not largely attended. For one thing, there were at that time only about four people in the know at all; for another, a German sentry was standing on guard immediately outside the door. Two officers in orderlies’ clothes were responsible for the whole operation. They removed _the whole of the partition_, loosened the two necessary planks and replaced it. The structure of planks fitted very closely against the side and top, except for one place at the top of the plank nearest to the corner post of the partition next to the cellar floor and immediately under the concrete of the staircase, where there was a small aperture looking like a misfit of the boards. Just under this aperture—and on the inside, of course, of the partition—the bolt was fixed. A small hand could just reach the bolt comfortably from the outside and slide it in and out of the corner post. Had the aperture been ever so little smaller, no male hand could have got in at all, and, in the absence of female society, the conspirators would have had either to give up this entrance altogether or increase the size of the aperture, which would have been most dangerous. By using this door as a means of entrance to and exit from the chamber which, as will be explained later, proved to exist behind the planks, the original party of conspirators succeeded in beginning a tunnel. They dug through the southern foundation wall of the building, turned east at right angles and succeeded by about Christmas in reaching a point beyond the outer wall[7]. A square chamber was made at the far end of the tunnel, then about 15 yards long, to receive the earth of the roof on the occasion of the escape, and all was ready for a move when Niemeyer suddenly put a sentry _outside_ the outer wall, almost on top of the proposed site of exit. [Footnote 7: Point _Q_ in plan on p. 53.]