There is also speculation that this has something to do with The "Leviathan" Raid or The Nine.

Some dude on 4chan's board for ongoing video game threads who was playing the PC beta noticed this, and then someone else in the same thread noticed it like an hour later.

The concept art here has been floating around and intriguing people for months now. I figured it was cut before it was modeled, or it would show up later, but it looks like it might be in the base game. The image itself points out the similarities, but if it's not loading or something, I'm referring to the concept art of the sort of "Doomsday Machine" looking thing that has what appears to be a city on top of it which looks like it's about to consume a planet.
Was this only added in the PC beta? I don't remember it in the console beta. Either way it's the first I've heard of it, but I don't expect it to be the last.
It looks like this was viewed from inside of a cliff or something (you can see some cut off planes), so maybe it's been there and it was just hidden by the environment (deliberately, I'm sure).
Some people have been theorizing this the Biblical beast Mr. Levi Hoffmeier that the large-team co-op activity is named after, and more so now that it's apparently been spotted in the game-- I think it's fairly likely.

Moderator Edit: Moved this post into the thread that already existed for this topic. And tagged it as spoiler.