User blog comment:Speculation no Kyojin/Episode 25 - The Titan in the wall/@comment-112.198.77.163-20131001161054

this one is a childish theory. Long time ago, titans and humans live together peacefully then one day, a bad faction of titan arises, leaving the good titans protect the humans... hold hands together and form three circles, which became the walls, I think each of the good titan leaders of different circle walls had names, mainly, titan maria, rose, and sina... in the story of the miner, the miner cannot penetrate to the wall sina because of the continuous growth of the titan wall underground, of course the more the titan grows, its weight increases, so instead of going up, it digs underground creating another wall below like bedrock.