Select a GTR-1 and go to the San Francisco level. Drive towards the Golden Gate Bridge and head to the end of the bridge. Stop at the very end, where the orange rope metal goes into the ground. Face to the left side of the bridge (if you were facing the tunnel at first). Notice where the metal rope goes into the ground. Back up where that metal rope is located, then back up against the wall/guard rail. You now will see the right side of the Golden Gate Bridge. Make sure your car is on the right side of the metal rope, about five feet away from the rope, opposite of the tunnel. Hold reverse and wait five or ten seconds. You will drop under the bridge.