Alright, so basically all i wanted to do was put a theory on the table. For those of you who have seen the Halo 3: ODST ending on legendary, you know it shows the Prophet of Truth uncovering a Forerunner structure deep beneath the city. I don't believe the structure is part of the portal to the ark because a legendary cinematic wouldn't be wasted on revealing something we've seen the center of all through Halo 3. Instead, i was pondering that under the entire Earth's crust lies this structure. What I'm saying is, could Earth be a shield planet? Or partially made into one? The humans did survive on earth during the firing of the halo array's by the forerunner. As we know from the terminals in Halo 3, the librarian had become fond of the human species, and, as such, could have ordered the construction of a shielded area for them to survive in. How else did they intend for humans to survive as their successors.