Well, I wouldn't say it's one of the Halo's, because when 343 showed all 7 of them up on a holo-pad thingy, none of them seemed special, except one, which had a a line coming off of it, and going to a box with blinking red letters in it, which I surmised to be the one destroyed in Halo: CE. I think it's pretty obvious that the structure is The Ark. You know- remotely detonates the Rings. The fact that it has similar markings doesn't make it a Halo, it just means that it was built by the Forerunners.
Pretty much I think they fired at the earth to uncover it making a crater, thats they the road ends and every thing is scorched and theres a parrellel/perpendical mt range, note that they make a circle curve on the side you can see.
That seems pretty likely, too. Because remember the (SPOILER ALERT) Series of concentric rings and the fins coming out of the side at the center of Onyx? looks kind of like that structure, don't you think? (END SPOILER ALERT)
and if you are crazy like me on that halo 2 soundtack there is a sub track called road to val or something which is now outside the present city of mombossa. and by the time of halo the two will be one. which gives the location of this structure.
maybe it is part of a halo, like th ark,
or where the forerunners most advanced technology was placed before they died.
or maybe that is where the forerunners live or died,
maybe albert einstine was a forerunner :Steve: :Steve: