Fans who are up to date with Attack on Titan remember the shocking revelation that there are Colossal Titans within the walls. Following this discovery came one of the biggest mysteries in Attack on Titan, why put Titans in the walls? In hopes of uncovering answers, Eren and the others continue their pursuit to Grisha Jeager's basement. It wasn't until episode 19, season three, "The Basement," that the answers were revealed.