D Point was the location of the Dark Kingdom's secret base located in the Arctic in the manga and anime.
D Point is a cold and featureless place in the arctic. As well as the Dark Kingdom's secret base, it served as their connection to earth. For much of season one, it remained unknown. The Sailor Senshi made efforts to locate it until the Sailor Senshi finally discovered it near the end of the season. It's here the final battle of the first season takes place.