Do the underworld glitch (at the Vinewood safehouse head north from Madd Dogg's) and fly to Area 69. Find the underground complex. Find the area where you get the Black Project, and you should see an empty long hollow room next to it. It should have red boxes at the top of the "silo". Fly underneath the empty silo then fly to the top of the silo. You should see a "cage" where the room is of the red boxes. Try to take off your Jetpack. When you do, you should land in the Black Project area or the rooms around there. To do the underworld glitch, find the safe house pool. Fly the Jetpack into the corner of the safe house. Then, fly to Area 69. Also, bring weapons and armor because the SAPD officers spawn down there.