From ancient-classical-history.yoexpert.com we learn that:
The Strait of Gibraltar is a narrow opening between the continent of Africa and Europe. It is located between Spain and Morocco and connects the Atlantic to the Mediterranean Sea. The mountains on either side of the strait are called the Pillars of Hercules because according to the legend of Hercules, it was created by Hercules in order to complete his tenth labor. The first nine labors of Hercules were based inside the Mediterranean rim, however, the tenth labor took him beyond the outer limits of the known world to a territory no Greek had ever seen.
For his tenth labor, Hercules went all the way to the Spanish Peninsula, where he had to bring back the cattle of King Geryon, a three-headed monster. Destroying the Geryon to capture his cattle was half the challenge. The other half was getting there. To reach King Geryon's cattle, Hercules had to venture beyond the Mediterranean Sea into the Atlantic Ocean, but one massive obstacle stood in his way: the mountain range that joined the continent of Africa and Europe and which sealed off the Mediterranean Sea from the Atlantic Ocean.
Hercules decided not to go around the mountain; he instead, went through it. He split the mountain into two with one blow from his sword, passed through the narrow strait, found Geryon's cattle and brought it back. This part of myth was created to explain how the Atlantic and the Mediterranean were joined. The cliffs on each side are forever linked to Hercules and were known by the Ancient Greeks as the Pillars of Hercules. The Northern pillar falls in the continent of Europe and is known as the Rock of GibraltarNo one knew, then, what was beyond them. To the Ancient Greeks, it was not only the gateway to the unknown; it was a portal between reality and myth. Having gone and come back only increased the reputation of Hercules.
They also tell us: It is also interesting to note that the vertical lines on he Dollar sign, which has its origins from the Spanish Dollar, represent the Pillar of Hercules.
www.britannica.com tells us the names of the twi pillars:
The northern pillar is the Rock of Gibraltar at Gibraltar, and the southern pillar has been identified as one of two peaks: Jebel Moussa (Musa), in Morocco, or Mount Hacho (held by Spain), near the city of Ceuta(the Spanish exclave on the Moroccan coast). The pillars are fabled to have been set there by Heracles (Hercules) as a memorial to his labour of seizing the cattle of the three-bodied giant Geryon.
