I have noticed a rather strange problem. I am using a polar projection (NSIDC Sea Ice Polar Stereographic), and added the National Geographic background layer. My problem is this: when you add it, it looks fine, but when I zoom in, I get this rather odd silvery ring of about 150km width around the edge of the base map (image attached). Does anyone have any ideas about why this could be? Is there an interaction with the NSIDC projection, or anything obvious that I might be missing?