Commentary Par IX 54

The word malta has caused difficulty.  Before Petrocchi, most texts capitalized it.  (There were at least six prisons in Italy that bore the name Malta.)  But it is also possible that Dante meant what Petrocchi thought he did (a generalized sense of 'prison').  If, however, he was referring to a particular place, most recent discussants prefer the choice of the prison for ecclesiastics situated in Lake Bolsena.