You'll need a jetpack to explore the Underworld.Gate#1: Go to the sea's cliffs east of Los Venturas near Sobell Rail Yard. Now fly along the ledge near water level. There is a glitchy rock wall that you can pass through in one point. The wall twitches and shakes. When you get in, you may have to fly north a little before it lets you get under fully.Gate#2: It's in a corner wall of your safehouse in Vinewood, right by the pool. Just walk into the wall at different spots until you find it.