Too get to the under ground your car should be the flame car(the car works for me) and start in casino place and in the plaza area. Now turn around and climb up the building. Now walk to the right on the roof(DO NOT JUMP OF THE RIGHT SIDE OF THE BUILDING). At the edge of the building turn around and turn into a car and drive off the LEFT side of the roof. You should be by a lake and go to the wall on the rihgt not the bridges and drive. Soon you should see the op of the car fliker of the water( this means its working). Soon you should fall into a black pit under the city to get out walk around till you hit wall and climb.