The Pillars appear as supporters of the coat of arms of Spain, originating from the famous impresa of the Holy Roman Emperor, Charles V, who was King of Spain in the years following the discovery of the Americas. It bears the motto Plus Ultra (Latin for further beyond), encouraging him to ignore the ancient warning, to take risks and go further beyond. It indicates the desire to see the Pillars as an entrance to the rest of the world rather than as a gate to the Mediterranean Sea. It also indicates the overseas possessions that Spain had.

http://en.wikipedia.org/wiki/Pillars_of_Hercules