Get a parachute and go to your safe house in Fort Carson. Save the game here to avoid having to get a parachute and traveling Fort Carson all over again if a mistake is made. Leave your safe house and go northeast to Bone County. Go to the Lil' Probe Inn bar. The Lil' Probe Inn is just north of the train tracks and directly across the road from a small mobile home trailer park. Lil' Probe Inn will display on the map area notification when you enter its area. Go inside the Lil' Probe Inn building. Look at the light hanging from the ceiling that is just behind the pool player. Jump towards the light as if you were going to climb it like a wall. It may take multiple attempts but eventually you will jump above the Lil' Probe Inn and be looking down at it. You will also notice a pitch black void surrounding it. Jump off the north side of the Lil' Probe Inn into the black void and immediately deploy the parachute. Press CJ's weight back so that he descends at the slowest rate. When you land you will be in the "Underworld". You will only see Bone County country-styled characters because the "Underworld" is considered as being inside the Lil' Probe Inn. To move around in the "Underworld", enable the "Spawn Jetpack" code. You can now fly to Los Santos, San Fierro, and Las Venturas with some difficulty, as obstacles from the normal world will still be there but you will not be able to see them. You must wiggle about a lot to find your way. In the cities, only a very small portion of them will be available. Using the Jetpack will make it easy to find the available areas. The "Underworld" has invisible jumps and falls, making the Jetpack necessary to navigate. The only way out of the "Underworld" can be found by going to Los Santos, where the small part of the city is available in the area called Commerce. Look for the building called Atrium that has two outdoor escalators and a top section with outdoor seating with umbrellas over tables. Once you enter Atrium you can then go outside be back in the normal world of San Andreas. It is probably a good idea to reset the PlayStation2 after visiting the Underworld and not save the game.