The infamous rock...

Alcatraz, what many thought to be inescapable. Men could get around the gaurds, but once they got around them, they had to find a way to swim a mile or so to land. The problem? Well the island is surrounded by a cold body of water. If you try to swim through it, you would probably end up dead of hypothermia. Besides the water being cold, there is also currents in the water. If you get caught in one, off you go into the ocean. Gaurds were also the top-notch gaurds. If they saw you running, they shot on sight.