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The basement (地下室 Chikashitsu?, also referred to as the cellar in the Funimation dub of the anime) is a location found beneath the Jaeger family's house in the Shiganshina District. It stores an important secret concerning the Titans, and Grisha entrusted his son Eren Jaeger with the key to unlocking this secret and the mission to reach the basement and discover it. To find the secrets hidden in the basement was one of the goals of the Scout Regiment during the expedition that led to the Battle of Shiganshina District.
The basement is a small room with a desk, a chair, and several shelves containing bottles of medicine and books about medicine. However, it was noted that this is just a facade meant to prevent suspicions from the Military Police in the case of inspection.
In the desk, there is a hidden drawer, opened with a key Grisha always kept with him and passed to Eren shortly before his death. Although the drawer seems to be empty, it has a false bottom, containing three books treated with peppermint oil and charcoal to keep moisture and bugs away.
The basement is located beneath the Jaeger family's house in Shiganshina District. It is accessed through a wooden door without a key, in the ground of the house which leads to a staircase. It seems to be adequately placed or protected against water as it did not flood despite the house being destroyed and remained abandoned for 5 years. At the end of the staircase, there is the basement, protected by another wooden door locked with a padlock. The whereabouts of the key for this padlock are unknown.
It is unknown whether the basement was built after Grisha Jaeger found a home in Shiganshina District or if it already existed by then. However, it is known that after Grisha married Carla Jaeger and they had their son Eren, Grisha would not let Eren see the basement. It is unknown if Carla knew what Grisha was really hiding in the basement.
At some point, Eren also asked Grisha when he was going to show him the basement, to which Grisha responded that he did not know, but maybe when Eren realized "the most important thing of all."
One day in year 845, after Eren expresses his interest in joining the Scout Regiment and seeing the world beyond the Walls, Grisha goes on a journey and promises to show him what he has been keeping secret in the basement once he comes back. However, later that day, Shiganshina is breached by the Colossal and Armored Titans and the district, along with the whole territory of Wall Maria are lost to the Titans.
Some time afterward, Grisha reunites with Eren and after learning of the death of his wife, he takes Eren to a forest and gives him the key he always carried with him, instructing him to never let it out of his sight, and giving him the mission to recapture Wall Maria and go to the basement, telling him that the power he is going to give to him will be useful then. Grisha then gave Eren an injection and allows himself to be eaten by him to pass onto him his Titan powers and the Founding Titan he had previously stolen, however, the events afterward are lost to Eren's memory, only recovering some sporadic flashbacks from time to time.
Five years later, after the Scout Regiment learn this and about Grisha's Titan powers, his attack on the Reiss family and the details of his past revealed by Keith Sadies that suggest that he came from outside the Walls, Commander Erwin Smith concludes that the basement is probably hiding the memories of the world that the First Reiss King erased from people's minds, and decides to lead the Scouts to Shiganshina with the purpose of finding the basement and discovering this secret.
The Scout Regiment reaches Shiganshina and successfully plug the hole in Wall Maria, but Zeke, Reiner, and Bertholdt ambush them and in the subsequent battle a great number of the soldiers are killed, leaving only nine survivors in the end. After the end of the battle, Hange Zoë, Levi, Eren and Mikasa Ackermann head to the basement and manage to find it. Hange is satisfied to see that the entrance was not flooded after years of abandonment, but when they descend the staircase and reach the door of the room, they are surprised to see that the key Grisha gave to Eren was not for its padlock, nevertheless, Levi destroys the door and they enter the room.
At first sight, the room seems like the regular workplace for a doctor, filled with bottles of medicine and books about medicine, but Hange deduces this is just to avoid suspicions from the Military Police, and Levi orders Eren and Mikasa to keep searching. Mikasa finds a keyhole in the desk which turns out to be the one opened by the key Eren has. They find a drawer which seems to be empty, but Levi discovers a false bottom containing three books treated with peppermint oil and charcoal to keep moisture and bugs away.
In one of those books, there is a picture of Grisha with an unknown woman and a child, all dressed in elegant clothes. The Scouts are astonished to see that the picture seems too elaborated to be a portrait drawn by human hands, but Eren reads a message written by Grisha on the back stating that this is a photograph, a technology unknown within the Walls, and that Grisha came from a civilization beyond the Walls, where humanity enjoys a refined existence; revealing that humanity has not perished as the Royal Government had made people believe.