The Magi Council Fall
In times gone past, a cloud city known as Una Bymatharg was established by a Council of Magi whose understanding of magic insisted there were 16 schools of magic and not 12, like the Cloud kingdoms had established.
But they were both wrong. As a result of the actions by a time-travelling group of young magi, a problem which was going to explode anyway, exploded earlier than it should have, and took out some of the support pilons for the Cloud city. As you may known from Cloud Kingdoms Cloud cities need pilons to stay afloat. so this one crashed into the sea.
It is reported (or rumoured) that the city still exists to this day, and its magic, if found, would train any decent mage into a grand high wizard.