Hospital underworld gateway in San Fierro
Go to San Fierro's hospital. There are two different buildings that are connected by a walkway that is over the road. Go towards the western building (hospital) were the cane is located. (where you get dropped off when you visit the hospital). To your left should be an ambulance. Go towards the ambulance, and face the east building "across the road". As you look across the road, you will see doors and a part of the building (hospital) that sticks out. Fly between the doors and the part of the building that is sticking out to reach the gateway to get in the hospital underworld. To get back to the real world, sink down to the endless pit. CJ will respawn at the closest street. You can also fly though the opening of the hospital where you first entered the underworld.