D Point in the anime
D Point was the location of the Dark Kingdom's secret base in the manga and anime.
Anime
In the anime, D Point was located in the North Pole. In the episode "Usagi's Awakening! A Message From the Distant Past," Sailor Venus and Artemis discovered the secret entrance inside the Cafe Crepe. A strange disturbance in the wall led Sailor Venus, Sailor Mars, Sailor Jupiter, Sailor Mercury, Sailor Moon, Artemis, and Luna into a cave that tunneled downward. Luna and Artemis got separated from the Sailor Senshi and eventually came out of the cave into a snow covered wasteland. They found a purple smoking crater and knew that it was the entrance to the Dark Kingdom. When the Sailor Senshi went to the North Pole the second time, they did so by Sailor Teleport. At D Point, the Sailor Senshi encountered the DD Girls. Sailors Jupiter, Mercury, Venus, and Mars were all killed in battles with the DD Girls at D Point.
Manga
In the manga, a disturbance in the form of a heat source was located in the Arctic Circle, which caused some suspicion. When Sailor Venus killed Queen Beryl, Prince Endymion - still under Queen Metalia's control - took her to the Dark Kingdom's hideout. Sailor Moon chased after them. Luna had trouble scanning the area when she tried to track Sailor Moon. When she was located at C Point, it was said to be 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 hideout underground. This was where the final battle against Queen Metalia took place.
Musicals
In Sailor Moon - Gaiden Dark Kingdom Fukkatsu Hen, Queen Beryl returned to D Point after her defeat and once more made it her base. She was eventually defeated there by Sailor Moon's Moon Princess Halation attack.