Levi and the rest of the Scouting Legion/Survey Corps are archeologists. During an excavation in the remains of the ancient Wall Maria they unearthed a huge crystal formation, which they then discover an encased body (Eren) after it was extracted and polished. The whole crew were ecstatic with their discovery, Hanji most especially. Somehow along the line Levi developed an attachment to the sleeping figure encased within. Link: http://snkkink.dreamwidth.org/2124.html?thread=2124620