In order to do this glitch you must be playing multi player. After you save cortanna and you just killed the power supply things, go to where the hole in the ground is but only have one person fall down. When the place starts crumbling have the person who didnt all down go to where they came in and just look out into nothingless. Ater a couple of minutes you should be able to see what the person that fell down the hole sees.