An incorrect mix of gases in the divers' tanks or human error may have been behind the tragedy in which five Italians lost their lives scuba diving in an underwater cave in the Maldives Thursday, an expert told ANSA Friday.     One of the unanswered questions concerns the gas mix in the tanks used by the divers.     Under certain conditions, even natural gases like oxygen can become toxic, observes Gerardo Bosco, full professor of Sports Physiology at the University of Chieti and director of