If you look at a map of the Levant you will notice Tyre is part of mainland Lebanon, however more than two millennia ago it used to be an island. It wasn’t a geological event which made the island disappear, rather war.
After defeating the Persians at Issus, morale was at an all time high at the Macedonian army. Alexander knew he would soon become pharaoh of Egypt, but there was just one small obstacle ahead.
The proud Phoenicians, lords of the Mediterranean, posed a thread as they could easily disrupt Alexander with their naval supremacy. So, on his way to Egypt, Alexander tried to get them to surrender. Byblos surrendered without a fight, Sidon too, but Tyre wouldn’t surrender that easily. Alexander almost talked them out of confrontation. But they could not come to terms on one issue: religion.
Few people know, but Alexander was basically as insufferable as it gets when it comes to being a religious zealot. He wouldn’t take a step without making a sacrifice to the gods, or build a temple or consult the priest or other things of that nature. So, if Tyre was to surrender a sacrifice to the gods was in order.
Inside Tyre was an ancient temple dedicated to Heracles. However, Arrian explains the Tyrians worshiped the Iberian Heracles, and not the Greek Heracles. Thus allowing the Greeks to worship their Heracles on the Tyrian temple would had been a terrible sin. The modern equivalent of this would be something along the lines of letting a Catholic worship Christ at an Orthodox church; yes, they too had disagreements over such things in ancient times.
And so, Alexander was not to be allowed to worship Heracles at the Tyrian temple, thus the siege of the island of Tyre began. The siege lasted 7 months, the Macedonians started building a rampant to connect the mainland to Tyre. Quite a feat.
One night Alexander had a vision, Heracles stood at the walls of Tyre where he called for Alexander to liberate the city for the Tyrians were heretics who did not workshop him on the Hellenic custom. Alexander’s idea of being on the right side of the crusade only got reinforced after this vision.
Tyrians who survived the siege were enslaved; all of them, a strong message for the surviving Phoenician city states. The rampant the Macedonians built was so large and wide that Tyre became forever attached to the mainland. Over time the rampant fully developed into habitable land. Today there is little physical evidence than Tyre was once an island.
And so, if someone asks you why Tyre is today a peninsula and not an island you can just reply: religious zealotry, a man who wouldn’t be told where he could or couldn’t pray as he so desired.