On the morning of July 21, 365 AD, people in Alexandria watched the sea disappear. Shortly after sunrise, the ground shook and the Mediterranean drained away from the shore. The harbor emptied. Ships sat tilted on the mud. Seaweed, rocks and stranded fish lay out in the open where water had been minutes earlier. Crowds walked out onto the seabed to see it. Some started picking up fish with their bare hands. Then the sea came back. A huge earthquake had ruptured under the sea near Crete. Modern geologists estimate it at magnitude 8 or higher, one of the strongest ever recorded in the Mediterranean. The tsunami it created spread across the eastern half of the Roman Empire, hitting Egypt, Greece, Sicily and beyond. The Roman historian Ammianus Marcellinus was alive when it happened. He wrote that the returning wave swept people away by the thousands, that huge ships were thrown onto the roofs of houses in Alexandria, and that others were carried nearly two miles inland. Years later, he said, he saw one of those ships himself near the town of Mothone in southern Greece. It was still lying where the wave dropped it, slowly rotting away far from the water. On Crete, the earthquake lifted the western end of the island by as much as 30 feet in a matter of moments. The old Greek harbor at Phalasarna, once full of ships, was left high and dry. You can still walk through it today, hundreds of feet from the sea. People at the time struggled to explain it. The pagan writer Libanius suggested the earth itself was grieving for Emperor Julian, who had died two years earlier. Christian writers told stories of saints who stopped the waves by making the sign of the cross on the beach. Alexandria never forgot. For more than 200 years afterward, the city held a yearly commemoration on the anniversary. They called it the "Day of Horror."