The magnetic lines of force emanate from the inner circle of the opening as if they were an inner curtain. This accounts for the "drunkeness" or confusion of the compass so often observed by Arctic explorers as they approach the opening itself. They never understood why, of course.
On page 216 of his book Farthest North Nansen
described the confusion of the compass: " To give the course of the drift (longitude) is a difficult task in these latitudes as there is a perceptible
deviation of the compass with every degree of longitude as one passes East or
West."
The above drawing is from Sadek Adams' book Hollow Earth Authentic.