"D Point" is the location of the Dark Kingdom's secret base located in the Arctic in the manga, Crystal anime and musicals.
Manga and Crystal
Disturbance in the form of a heat source was located in the Arctic Circle. Sailor Venus eliminated Queen Beryl, and Prince Endymion wash still brainwashed by Queen Metalia took her to the Dark Kingdom's hiding place and Sailor Moon went after them. Luna had difficult locating D point in order to find Sailor Moon and when she finally did locate Usagi, it was said to be at C Point 72°N latitude. Luna lost them at D Point, which was in the middle of the Arctic Ocean. Luna then located Sailor Moon in the Dark Kingdom's hiding place and this is where the battle with Queen Metalia took place.
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.
Musicals
In Sailor Moon - Gaiden Dark Kingdom Fukkatsu Hen musical, Queen Beryl returned to D Point after her defeat and once more made it her secret base. She was eventually defeated there by Sailor Moon's Moon Princess Halation attack.