"""Please correct me if I am wrong, but I do believe actual Hueys flew over for the opening of One."" 
 You are wrong - I was front and center in the mud pit for nin and metallica. when nin started their set it was mostly what i would call ""barely subdued enthusiasm"" and the crowd actually picked up people who fell down and protected them from damage, it only started getting pressed and dangerous when metallica started their set, people were actually getting trampled if they fell underfoot (and there were no helicopters flying over). my friends and i had to beat a hasty retreat to a few hundred feet from the stage when metallica opened up - at that time, people started going nuts. the pit was literally a you fall you die zone. 

 I do agree with you that the highest crowd density was when metallica played    

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