Mike Miller continues his Legendary Walkthrough of Halo 2; up this time is Quarantine Zone. Like the previous level, Sacred Icon, mostly he just runs right through it, and wonders why, with air transport available, the Covenant don't think of that themselves:

I guess there's no point in wondering why the phantom didn't just drop us off here to begin with. The Covenant probably execute their officers for asking questions like that.

Indeed. In addition, there is a really neat hiding place in the final gondola, demonstrated via QuickTime. Thanks Louis Wu at HBO.