Perhaps the reason why the ONI building is so important is that it is actually the location of ONI's excavation of the portal to the Ark. I can see there being a large shaft that leads underground to the excavation site, and ONI not wanting the Covenant to learn about this mysterious artifact. It would make sense, as there is nothing to suggest otherwise. This could be Dare's mission. She would have to blow the building up, which would cover any archeological sites up with rubble. [b]Support[/b] -The denotation "Alpha Base" may imply this was the first Forerunner artifact found. This is very likely, as it is located on Earth, which would mean Humans could have found it before they achieved Faster than Light capabilities. -The ONI base was located on the water, and in halo 3, specifically on the level "The Storm" it is obvious that the location was at one point covered in water as evidenced by the Boat that had washed ashore. -ONI would not have uncovered the the entire portal just scratched the surface. -For those who argue that this is not possible because the portal is located in Voi, firstly, no it wasn't, and even if it were, you have to remember that by the time of Halo, Mombasa has greatly expanded. The current city of Mombasa is known as Old Mombasa in Halo, while New Mombasa stretched for miles upon miles. And besides, if you notice in the ending cutscene of "The Storm", not only is there a great distance between yourself and the portal, but the portal itself is huge enough to encompass tens of miles. [Edited on 06.20.2009 6:22 PM PDT]