Pillars of Hercules

The Pillars of Hercules are the gateaway into the Mediterranean Sea or Mare Nostrum, that are guarded by Hercules. They are thoughtto be the Rock of Gibraltar and, it is debated, Monte Hacho in Ceuta or Jebel Musa in Morocco.

Mark of Athena
Leo Valdez steers the Argo II into the entrance of the Mediterranean Sea. However they cannot go any further as an island shimmers into existence. Hercules is seen standing on the beach of the island.

After Jason Grace and Piper McLean return from the island, Leo steers the trireme over the island and pillars into the Mediterranean Sea.

Appearance
The Pillars are two large white columns, that are as big as the masts on the Argo II. They are carved in a Grecian style.

The pillars had a warning engraved on them, but to Piper it looks like the words are inlaid in the sand, in Latin, nec plus ultra which means 'nothing further beyond' as for the Greeks and Romans, it was the end of their world.

Trivia

 * The Pillars of Hercules are the two parallel lines of the dollar sign ($), also known as cifrão. The Pillars of Hercules featured on the Spanish coat of arms, which were then used on Spanish dollars, from which the American dollar was derived from.