How do you unlock area 69 in gta sa?

In GTA: San Andreas, Area 69 really becomes accessible with the mission, Black Project. In this mission, The Truth persuades CJ to sneak into the Area 69 inner laboratories and retrieve the secret that is hidden within this secret government location. Once CJ has obtained the item and the mission is completed, the interior of Area 69 is no longer accessible. Only the compound of the location can be entered at any other time, and then only with extreme danger (and with 5 stars). Apart from the soldiers, there are missiles to shoot down anything flying over or near the area. But it can be accessed with care and attention, and the tank that is hidden there can be taken and driven away and saved in the hangar at Verdant Meadows!