Abyss of Time

The Abyss of Time is an massive underground labyrinth that appeared in the playable epilogue in Persona 3: FES, titled The Answer. Originally a state of hibernation, it became 'active' when it was attracted by the grief and sorrow of SEES, and spread it's influence there, rooting down below the dorm.

Profile
When the phenomena of Dark Hour and Tartarus was created, Tatarus, growing out of the ground each night eventually left a great cove underneath it's roots. Following the death of the Protagonist, the Abyss of Time was attracted by SEES' grief and sorrow, and spread its influence there, rooting down below the dorm. Eventually the Abyss of Time would become active, locking SEES in an infinite time loop where the time will re-reset itself whenever the clock hits 12:00 A.M. ,leaving the time forever frozen in March 31st.

The Answer
Following the events of the Protagonist' death, Mitsuru Kirijo decides to shut down the dorm, while calling the remaining SEES members to regroup one last time prior separating their paths. During the celebration, they found out that they are locked in an infinite loop where the day kept staying in March 31st. As their confusions arise, SEES was attacked by Metis, an anti-shadow weapon who claims to be Aigis' sister.

Eventually Metis was subdued, and from Metis SEES learned about the phenomena of the Abyss of Time. Metis first subject to destroy SEES, so the Abyss of Time can be neutralized, as she believed that they are the main problem of the existence of the Abyss of Time. SEES would later decide to take on their Evokers one last time again, to investigate the Abyss of Time.

Throughout their investigation, they encounter a recurring silhouette resembling the Protagonist, but upon reaching near him, the silhouette flees, leaving powerful Shadows to halt SEES' advance. Apart of the recurring silhouette, SEES also encounters distinct respective floor which reveals each members' past:

Ken Amada's memories retold the time where his mother was murdered by Shinjiro Aragaki. He was taken to the Police Station for questionings and investigations, but the Policemen did not believe his story of a 'Monster' destroyed his home and his mother.
 * Ken Amada

Akihiko's memories retold the time when he was still a sophomore in Gekkoukan High School, a young, talented boxer, his talents immediately gained the attention of other school's staff, and was invited to transfer. Akihiko instead declined, and met Mitsuru Kirijo for the first time. Mitsuru coerced him into joining SEES, whom he eventually agreed to.
 * Akihiko Sanada

Junpei's memories retold the time prior the Protagonist' first encounter with the Arcana Magician. Escaping from his alcoholic father, Junpei ended up in Paulownia Mall. In there, he met Koromaru for the first time, and eventually, during the Dark Hour, he was attacked by Shadows, which eventually motivated him to join SEES upon Akihiko's invitation.
 * Junpei Iori

Mitsuru's memories retold her first encounter with a Shadow, and the subsequent awakening of her Persona. Still an adolescence, she joined her father, Takeharu Kirijo in investigating Tartarus. During this time, one of their bodyguards turns into a Shadow (one of them comments that they were warned about "artificial potential" hinting they were part of the experements that gave Strega their artificial personas). The other bodyguards protecting them are killed. Mitsuru eventually managed to summon her Persona for the first time under the urge to protect her father. However, Takeharu lamented that because of his work, Mitsuru was forced upon taking the task of cleaning the previous generation's mess.
 * Mitsuru Kirijo

Yukari's memories retold in her first enrollment into Gekkoukan High. During her enrollment, she received a letter of father, Eiichiro Takeba, which tells her to be happy. Yukari was later invited to join SEES by Mitsuru, whom she agreed so she could understand the cause behind her father's death.
 * Yukari Takeba

The door leading of Aigis' past was not a part of her memory, but rather a recurring dream of hers after the Protagonist' death. In her dream, she saw the Protagonist, however, she was unable to catch him, with the Protagonist leaving her further and further away (if one looks closely near the end of this dream, they can see a silhouette of Metis, hinting at her true orgin). In there she confessed her will of losing the side of humanity and the penalties a living being must face upon the Protagonist' death.
 * Aigis

Upon venturing to the deepest area of the Abyss of Time, the recurring silhouette of the Protagonist is revealed to be a manifestation of SEES' grief and their wish to see the Protagonist one more time. Making his entrance, the silhouette challenges SEES to a battle, retaining his Wild Card ability and several of the Protagonist' movesets. When the silhouette was defeated, SEES was granted with keys which allows them to travel time, however, each members have their own wishes to use the key, and neither of them wanted to give in.

As their opinions differ, SEES was separated and fought each other in the Abyss of Time, with Aigis and Metis reigning as the champion. Fusing the keys into the 'true key', Aigis decided not to change the past, but wished to know what is the cause of the Protagonist' death. However, due to the time of spending in the Abyss of Time, the labyrinth began spreading it's influence, cracking the floor of the Dorm. Noticing that they don't have much time left, SEES decided to return to the time where the Protagonist seals Nyx via the final door in the Abyss of Time.

SEES eventually realized that the Protagonist used up his life essence to conjure the Great Seal, however, his intentions was not to seal Nyx, but Erebus, and grand manifestation of humanity's grief from contacting Nyx to bring forth The Fall. Noticing SEES' presence, Erebus became hostile and attacks SEES. Eventually SEES was able to defeat Erebus, with their grief and confusion lifted, the Abyss of Time was destroyed as well, using the true key to unlock the dorm's door, time finally flows again.

Floors
Similar to Tartarus, the Abyss of Time is divided into doors, each floor bearing distinct design: The final door, upon forging the 'true key' will reveal itself in the center of the Abyss of Time, which leads to the past where the Protagonist performs the miracle of sealing Nyx.

Trivia

 * The background music of the Abyss of Time, Interstice of Time is played in a somber fashion, but becomes more and more lively when SEES investigates deeper in the Abyss of Time.
 * Aigis can request Fuuka to change the background music.
 * 4 selective tracks during her investigation of the Abyss of Time.