Alcatraz was one of the most successful prisons ever built in terms of preventing prisoners from carrying out successful escape attempts. Although the prison itself was at the time one of the most secure facilities in North America it was the icy water of the San Francisco Bay that prevented prisoners from reaching freedom. If the prison was so successful, why was it closed?
During the 29 years Alcatraz was a Federal prison it held claim to an almost 'perfect' record, 36 prisoners made 14 escape attempts with two men trying to escape twice. Of those 23 were caught, six were killed from gunfire during their escape attempt, two drowned and five are listed as "missing and presumed drowned". The only reason Alcatraz can't claim a 'perfect' record is because there is a small possibility that the five "missing and presumed drowned" are still alive, although this is unlikely. Alcatraz held many famous prisoners from Al Capone to James Bulger, usually prisoners sent to Alcatraz were ones who presented a serious security risk or who had caused trouble at other prisons. One prisoner held there that I found interesting was Bumpy Johnson who was depicted in the Ridley Scott film, American Gangster.
So why if Alcatraz was so secure and a prison capable of holding the most dangerous inmates was it closed? Ironically salt water forced the closure of Alcatraz, the very ingredient that made the prison so secure in the first place.
Alcatraz used salt water to flush the toilets, salt water contains magnesium chloride, sulfate ions and hydrogen carbonate ions that will attack concrete to a certain degree. Although what really starts to corrode in a concrete structure is any of the steel substructure within, the main line of defence in preventing prisoners from tunneling through concrete walls. Usually the steel inside concrete will react with it's interior alkaline environment, this forms a film that protects the steel. However when salt water soaks into concrete the chloride and sulfate ions weaken the film, once the film is breached the corrosion process begins to work on the steel itself.
Over time the concrete walls of Alcatraz became so weak that it was possible to tunnel through them with only a spoon, three prisoners did just this to escape in June 1962. Their bodies were never found and as a result the US Marshall’s office is still investigating the case, a case which will remain open on all three until their 100th birthday's. After that attempt in 1962 it started to become apparent that Alcatraz's walls were no longer secure enough for a Federal penitentiary, that and it was over three times more expensive to operate than the average US prison ($10 a day compared to $3 a day). As a result Alcatraz was closed on March 21st 1963.
It seems ironic that the very thing that made Alcatraz so secure was what ultimately led to it not being secure enough.