Another Easy San Fierro Blue Hell Entrance

In San Fierro, go to the center of 'Downtown' to a place called 'Financial' and find the building where you can do the Valet Parking side-job. Enter the underground parking deck under the vertical PARKING sign, and before you get into the open parking area there is a section of ceiling you can get through using a Jetpack or the Seasparrow. You can see it clearly in this image. It's the section of wall that's white where the median starts on the ground.