The North Pole is at the Head of the lake in Winter Holiday. It is the the Old Viewhouse or Summerhouse where the old man who built it just over a hundred years ago watched the Lake in all weathers and all seasons (WH26).
The North Polar Expedition plans to travel to the North Pole. Nancy says .. nothing ever happens in the winter time ... so we started a Polar expedition. Peggy adds as soon as we could we'd go to the North Pole over the ice .. (WH3).
Peggy says that It's right at the head of the lake. The extreme north of the Arctic, and only a few yards off the ice. You can’t mistake it (WH23). Later Nancy sees a light lower down than the lights of the village where there is nothing except ..... and then sees the Ds long and short flashes reading N.P. (WH25).