There's a large floating metal construct high above the sky, presumably too high for any airship to reach. It could be one of the three towers. Secondly, when people die their souls turn into birds of red light and these same birds feature prominently in the opening scene of Ar tonelico II: Melody of Metafalica. The guess is thus: the actions of Alan and the Shadow Lands damage the planet's Heart of Gaea and a program was thus created to ensure that everyone was evacuated to one of the towers.
- Jossed. The birds in the game's opening are only symbolic representations of the power of Songs, and anything that goes below the Sea of Death disintegrates, no ifs or buts.