Criss cross

Most people know of the phrase "crossing the rubicon" meaning to take an irrevocable step, and you probably know that it comes from ancient Rome and refers to Julius Caesar and his army crossing the river Rubicon, after which there was no turning back.  But where is the Rubicon?  And the surprising answer is - nobody's sure.  After the fall of Rome, map-making became of less and less importance, and just which river was called what became confused.  So we know roughly where it is. but precisely - no.