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

Originally Answered: Attack on Titan (manga): What is in Eren's Basement? It was answered in chapter 85 of the Shingeki No Kyojin Manga (Spoiler alert), there they found a series of diaries and photographs that Grisha Yaegar, Eren´s father owned.

Eren and Mikasa opened the drawer and found a false bottom hiding three books – Grisha's journals. These books hold the true history of the world, a history that was not taught in their schools. Their new history lesson began with a photograph of Grisha accompanied by a woman and a young boy.