The end of the earth. That’s what the Straits of Gibraltar were to everyone except the Phoenicians. People were terrified to go through them, and for good reason.
The waters of the Straits are shallow, the passage is narrow, and the side toward the sea hid a covey of Phoenician war ships that sank everything that wasn’t Phoenician. They had a closely-held, strictly enforced code that the Straits were the door to the Phoenician’s livelihood, and passing them, or divulging the secrets beyond them, including the sailing by longitude, was punishable by death, even to the Phoenician sailors.
The war ships sank anything that wasn’t Phoenician. For the ships that were sunk, it was the end of the earth. Ships that went out were sunk far enough away so they were out of sight of land. No one saw what happened to them. They just fell off the edge of the earth, and the sailors encouraged the myths about sea monsters and flat earth with it’s drop-off to discourage exploration. Quite successfully, it can be said.
The Atlantic Ocean belonged to the Phoenicians, and also the Indian Ocean all the way around India to China for hundreds of years. That monopoly only fell apart when the age of empires began building navies that were strong enough to overcome the city-states of Phoenicia, and even then it took several hundred years to wrest the supremacy from them.
The Straits of Gibraltar only gave up her bad reputation as the Gates of Hell reluctantly, and then after a very long struggle.