Near the hospital in Staunton Island is a small construction site where the player can find a ramp and jump to a platform. The player can go inside the glass of the windows in a corner and will fall into a tunnel (the same tunnel seen during the mission "Bringing the House Down"). During the mission "Bringing the House Down", the Blue Hell under Fort Staunton is solid so the player won't fall through (Tested with the use of Cheat Device). See Carson General Hospital for a full description.
(Note: You need to activate the 'Perfect Traction' cheat to perform this glitch). In front of Sweeney General Hospital at Portland View there is a place where there is an Angel and an Esperanto. Get a car, jump using 'Down', drive it to the taller roof, get out of the car or drive it and jump to the white lanes. You will fall into the Blue Hell and will re-spawn at the nearest point at Portland View or Chinatown.
(Note: Activate the 'Cars in Water' and 'Perfect Traction' cheats to perform this glitch). Go to the Shoreside Lift Bridge. Drive straight while the bridge is still lifted up. Go to the Shoreside Vale end. Jump and drive under the bridge (before a pillar) and when you have jumped in the bridge, go to the grass. You will fall into the Blue Hell and re-spawn later at the Francis International Airport end of the Porter Tunnel.