If you've ever played GTA: San Andreas for real, and not as a jetpack cheat code to go to the airport, you will notice a restricted area on the map, which is called Area 69, a simulation of Area 51 in Lincoln County, Nevada. You can enter the Area 69 top secret area's labs. There may be spoilers in here, so do not read on if you don't want your game spoiled. Follow the simple steps below
Go to the large building east of the Las Venturas Transfenders.
Go to the south side of the building where the entrance is. The doors are not solid.
Spawn a jetpack (Left, Right, L1, L2, R1, R2, Up, Down, Left, Right), and jump into the entrance, which will lead you to the world known as the Blue Hell. Try not to go to low in here.
Fly under the state until you reach the Restricted Area. If you get a wanted level, just ignore it, they can't get you here. Navigate through the labs until you see a long corridor that has a grate for a floor.
Find its start in an atrium near it, and the small entrance to the waterway below the grate.
Fly into the labs by going through the ceiling directly above the grate opening.
Feel free to explore all over the labs!