The concept of prisons as we know them today is relatively modern. In antiquity, jails served less as places of penitence and more as a purgatory before the final judgment of guilt, which was often punished either by enslavement or execution. Before the mega jails and super-maxes of today, historical prisons took on many forms, from isolated islands to underground dungeons. Excluding any prison that is currently open and also the horrifically depressing sub-genre of concentration camps, this list reveals some of the scariest prisons in history.