Basement

A basement (地下室) is found beneath the Yeager family's house in the Shiganshina District. This place was supposedly used by the head of the family Grisha Yeager for his work as a doctor; however, after the fall of Wall Maria, it was revealed that it actually stored an important secret concerning the Titans, and Grisha entrusted his son Eren Yeager with the key to unlock this secret and the mission to reach the basement and discover it. To find this secret was the mission of the Survey Corps during the 58th expedition beyond the Walls.

Description
The basement is located beneath the Yeager family's house in Shiganshina District. It is accessed through a wooden door without 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 remaining 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 case of inspection (oddly, there is a bulb in the ceiling, despite the civilization within the Walls not using and apparently not even knowing the electricity).

In the desk there is hidden drawer, opened with a key Grisha always kept with him and which 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.

In one of those books it was found a photograph (a technology unknown by the civilization of the Walls) of Grisha with an unknown woman and child, all dressed in elegant clothes. The back of the photograph has a message written by Grisha revealing that he came from a civilization beyond the Walls, where humanity enjoys a refined existence, and that humanity has not perished as the Royal Government had made believe.

History
It is unknown when the basement was built, and whether it was built after Grisha Yeager arrived to the Walls in year 830 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 his 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". The meaning of these words has not been revealed.

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 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 if his sight, and giving him the mission to recapture Wall Maria and go to the basement, telling him that the powers he is going to give to him will be useful then.

Grisha then injects Eren with Titan serum, triggering a transformation into a mindless Titan, and allows to be eaten by him, passing him his Titan Shifter powers and the Coordinate 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 shifting powers, his attack to the Reiss family and the details of his past revealed by Keith Shadis that suggest that the 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 the 58th expedition beyond the Walls with the purpose of finding the basement and discovering this secret.