imagine if you could get in the traveler (that floating globe thing) and sneak past the guards and pilot that thing
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This wouldn't work for a few reasons. First, the Traveler is to big to 'sneak' around in. Everyone could see you for miles around. Besides that, if the Traveler moved, then the City would fall to the various enemies who had been held back by its power. If it did happen, the mystery about the Traveler would be gone, and that would be like revealing everything about the Forerunners in Halo.