That is a wild warning about the atoll! I wonder if it is from seismic activity kicking up radioactive material left from all the testing in that region. That entire region is a ticking time bomb in so many ways.
I just found that the SPACE FENCE radar system is there. The most advanced radar apparently. The NOTAM is for the area to the north of the building on the island and is probably to do with the radar.
I like using google earth to explore terrain in an area. It is pretty good for following waterways to there sources and such.