The tunnel beneath the National Mall between the Castle and the natural history museum exists (chapter 16). The only difference is I added a bend in the path. In reality, it’s a straight line for 730 feet. The spiral staircase winding through the north tower of the Smithsonian Castle is there (chapter 19) and its provenance, as detailed, is accurate. I made one change, adding an exit on the second floor, which no longer exists.
