Historical clue formulations

• Hard to see through — 2016-09-05
• Impossible to see through — 2014-04-07
• Hard to grasp — 2014-03-22
• Clear as mud, so to speak — 2013-05-10
• Impossible to see through — 2012-10-16
• Unclear — 2011-10-30
• Not easily understood — 2009-03-14
• Unclear — 2003-07-10
• Opposite of transparent — 2001-06-25
• Hard to understand — 2001-06-16
• Hard to understand — 1999-08-11
• Unclear — 1998-11-22
• Muddy — 1998-11-14