Dark Ocean

The Dark Ocean, also known as Quantum Sea is a world in Digimon Adventure 02 and Digimon Adventure tri. that has the power to bring twisted thoughts to life, magnify and corrupt them, and inverse the natural order of things. It is ruled by Dragomon and plays a central role as one of the world's three main dimensions, along with the Real World and the Digital World.

The History of the Dark Ocean
During his solo trip, Matt, realizing that he has not been the best friend or the best brother, becomes depressed and is transported to the World of Darkness, where he is trapped in a dark cavern. However, Gabumon makes him come to his senses, freeing him from that site and returning him to the Digital World.

While searching for Matt, Sora is also depressed, getting trapped in the same cavern. Even when Matt and Joe go on their quest and try to help her, Sora becomes even more depressed, until she sees herself in the deepest part of the Dark Ocean. Eventually, Matt and Joe are able to get her in and free her from the Dark Ocean

Yukio Oikawa guides Ken Ichijouji to the Dark Ocean after the death of his older brother, Sam Ichijouji. Ken dips his Digivice in the Dark Ocean's water, transforming it into his black D-3 and spurring the creation of the Digimon Emperor persona. Kari Kamiya is also pulled into the Dark Ocean due to holding too much inside her. She comes across Scubamon bound by Dark Spirals and frees them once rescued by Takeru "T.K." Takaishi, Patamon, and Gatomon. They only reveal their true forms as dark sea creatures who wish to mate with her so that they can produce powerful offspring to fight the new god (In the English Dub, the creatures want her to become their new Queen who lead them in the battle against the Dark Ocean's ruler). Angewomon drives them off, and they return to the ocean's depths.

Some time later, Ken, in his Emperor personality, decides to go to the Dark Vortex, which is presumed to be a point that communicates the Digital World with the World of Darkness, to collect Devimon's data.

After Ken begins to atone for his actions, he falls into despair and darkness due to the influence of his inner dark self while searching for Gatomon's lost Holy Ring. Kari follows suit, with both suffering intensely until Yolei Inoue, who was dragged in after them, snaps them both out of it. In the latter's case, it triggers the DNA Digivolution of Aquilamon and Gatomon, merging them into Silphymon and defeating the Control Spire-built Blossomon sent by Arukenimon and Mummymon. hen he proves too strong for the DigiDestined to defeat and destroy, even with their combined might. By combining their powers, they trap Daemon in the Dark Ocean, finally putting an end to the threat he posed to Earth.

Three years later, after the reboot revived her Tapirmon, Maki Himekawa becomes stranded in the Dark Ocean after her Tapirmon failed to remember her. She had brought a bazooka with her, and uses it to defend herself against the Scubamon. After successfully defending herself, she drifts away in the ocean.

Trivia

 * Sunny Town, the town of the Zombies Pony of the MLP fan-game, Story of the Blanks, it's really a place of the Dark Ocean.