The Great Collapse was a major disaster in the city of Winterhold in 4E 122. Shortly before the collapse, the Sea of Ghosts came alive as monstrous waves battered the city destroying entire districts. When the city collapsed into the sea, most of it was lost, save for the Jarl's longhouse, a few surrounding houses, and the College of Winterhold, which was allegedly protected by magical means, leading some Nords to suspect the College could have either been involved in the disaster or could have prevented it.
Nobody is certain of the true cause of the disaster, though some believed the eruption of Red Mountain had far-reaching consequences that were only felt years later.