"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 the majority of the season, its location remained unknown to the Sailor Senshi. The Sailor Senshi found a portal to a series of tunnels near the season ending, which had a run-off tunnel; Luna and Artemis traveled through the run-off and found themselves in the Arctic. After an unknown time of searching, they located D Point.
The next episode, the Sailor Senshi used Sailor Teleport to teleport directly here. The DD Girls volunteered to confront the Sailor Senshi, their powers being strongest here. However, while the DD Girls were destroyed, the Sailor Senshi also died aside from Usagi; she was confronted by the ghosts of her friends, who encouraged her to go. She ran towards the Dark Kingdom with newfound courage.
In D Point, there was a purple crater which emitted smoke, where the Dark Kingdom's operations throughout the entire season thus far were conducted.
It was (presumably) destroyed after the final battle against Super Beryl.
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.