In Skyrim, what caused the Great Collapse that sent most of the city of Winterhold into the ocean is never revealed, but it might have been accidentally destroyed by the protagonist, the Dragonborn. The theory might seem implausible at first, but it's not an impossibility, and may be more credible than it appears at first glance. Developer Bethesda has neither confirmed nor denied it, and until they do, it's up to player interpretation.
The Great Collapse in Skyrim occurred in the year 4E 122, approximately 80 years before the start of the game. A series of powerful storms struck the town of Winterhold, leading to the swift erosion and collapse of most of the city into the Sea of Ghosts. Only a small part of the city survived, including the College of Winterhold, one of the most unique locations in Skyrim. Winterhold never recovered from the Collapse, leaving it a tiny town with little residents and less visitors. Since the College was left unharmed, many of the locals blamed its mages, along with its Dunmer refugees. According to the College, it was caused by the volcanic eruption of Red Mountain, yet this event occurred in the year 4E 5, over 100 years prior.
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Most of the Dragonborn's interaction with Skyrim's Winterhold centers around the College of Winterhold questline, where they join, learn magic, and eventually become the Arch-Mage, uncovering an ancient and powerful artifact known as the Eye of Magnus in the process. It quickly becomes unstable due to meddling from Thalmor agent Ancano, who goes mad and almost destroys all of Tamriel in his lust for power. The Eye of Magnus is key to the theory that the Dragonborn accidentally caused the Great Collapse
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