Pillars of Hercules
Real World Background
The Pillars of Hercules is an old name for the Straits of Gibraltar, the passage from the Atlantic Ocean to the Mediterranean Sea (and vice versa). In the classical world, this passage was believed to be the limits of the world, beyond which one should not venture. The Pillars were so called because, according to Greek myth, the demi-god Heracles or Hercules set up these pillars during his twelve great labors.
The Rock of Gibraltar (another of the Stone of Destiny's names) dominates the Straits of Gibraltar.