This really does work and it only took me one try to do it!!! All u have to do is spawn an annhilator go to the second tallest building in liberty city. Once on spawn a motorcycle half inside the building half out. Get on the motorcycle and drive barley inside the building then spawn a car any car. Then drive off the side and u should land under the city. Sometimes it may appear u back on the street though but that usually only happens when ur on foot or an a motorcycle. so good Luck!!