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.
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.
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 afterwards, Grisha reunites with Eren 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 the events afterwards are lost to Eren's memory.