The island of Maria Madre in the Pacific is surrounded by shark-infested waters. It is also a Mexican federal prison. 800 men and 100 women, carefully selected by the administration, serve the rest of their sentence, together with their families if they wish, in a semi-free state. This open-sky prison challenges the traditional penitentiary : convicts prepare for their release through job training and unpaid, obligatory work. Although this is a costly experiment, it is an ‘investment’ since the rate of multiple offences among prisoners from Maria Madre is well below that of the continent.