Basement

Grisha Yeager's basement (地下室) is beneath the Yeager family's house in the Shiganshina District. Supposedly, it contains materials for Grisha's his work as a doctor; it actually contains 3 books that reveal the secret of the Titans, Grisha's past, and the world outside the Walls.

Grisha kept the basement locked from his son Eren, but after the fall of Wall Maria, Grisha gave his son the key and entrusted him to retake Wall Maria and reach it. To find the secrets hidden in the basement was one of the goals of the Survey Corps during the expedition that led to the Battle of Shiganshina District.

Description
The basement is located beneath the Yeager 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.

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 Brigade 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, and a photograph in one of them of Grisha with an unknown woman and child dressed in elegant clothes. A message in the back of the photograph reveals Grisha's origin and the true state of humanity.

History
It is unknown whether the basement was built after Grisha Yeager arrived at the Walls in the year 832 or if it already existed by then. However, it is known that after Grisha married Carla Yeager and they had their son Eren, Grisha would not let Eren see the basement, with Carla explaining that her husband's very important work tools were down there. 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."

Story
One day in year 845, after Eren expresses his interest in joining the Survey Corps and seeing the world beyond the Walls, Grisha goes to 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 District is breached by the Colossus and Armored Titans and the district, along with the whole territory of Wall Maria, are lost to the Titans.

After learning of the fate of the district and the death of his wife, Grisha 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 gives Eren a Titan injection, triggering a transformation into a mindless Titan, and allows himself to be eaten by him, passing onto him his Titan powers and the Founding Titan he had previously stolen from Frieda Reiss. The process causes memory loss to Eren, and he forgets the incident, only recovering some sporadic flashbacks from time to time.

Five years later, after the Survey Corps learn this and about Grisha's Titan powers, his attack on the Reiss family and the details of his past revealed by Keith Shadis 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 King erased from people's minds, and decides to lead the Corps to Shiganshina with the purpose of finding the basement and discovering this secret.

The Survey Corps reach Shiganshina and successfully plug the hole in Wall Maria, but Zeke, Reiner, and Bertolt ambush them and in the subsequent battle most of the soldiers are killed. When the fight seems lost, Erwin reveals that he still has a plan to win but does not want to apply it because he will die as a result, and he does not want to die without seeing what is in the basement, revealing that the reason he was able to make it that far was because he was waiting for the day he would get answers to his questions. However, on Levi's advice, Erwin drops his dream and puts his plan in motion, dying along with most of the recruits.

After the end of the battle, Hange Zoë, Levi, Eren and Mikasa Ackerman 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 Survey Corps 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 the people believe.

Titles of the three books found in the basement are later revealed to be: "The Early Life of Grisha Yeager," "The Extent of Our Knowledge of Titans and Their History," and "Information About the World Beyond the Walls."

Trivia

 * Despite the fact that the basement was made to appear nonsuspicious in the event of Military Police Brigade inspection, there appears to be an electric light bulb in the ceiling of the basement. This error was corrected in the Volume 21 release, where the light bulb is replaced with a gas lantern.

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