It is a large mechanism created by the Allagan Empire, says it in all the GC story lines. You can even tell that it is man made now after 1.22 when you look in the sky. Nael Van Darnus has been trying to pull it down but I'm unsure if he has succeeded or it is coming down on its own because he's been "feeding" it aether. The latter would be interesting; say it comes down, destroys the empire and becomes our new enemy. But alas we must all wait till that day comes, luckily that day isn't to far off.