Discovered by hitman2992 at PSP Crazy:
Here is a following quote:
"well next to the hospital at Staunton Island there is a construction site go in there jump up to the hospital roof wit one of dem pj 123 or wutevr that number is motocylces . then *slowly* drive through the wall on the right. then you find yourself in the unfinished tunnell . then explore a little find the opening out of the tunnell. and spawn a tank drive off out of the tunnel and it will glitch again putting u in the same tunnel wit a floating tank. the only way to get out of the tunnel is to kill ur self."