To get the access the secret underworld you must go into the Los Santos gym. Once you get in there, get the jetpack cheat and fly upwards in the dark corners of the gym. You will be on top of the gym able to see through the roof and surrounded by darkness. Fly off the edge and down to the underworld. It should be like a large white desert. If you walk around you can stumble upon weapons, armor and sometimes even invisible water.