The line of succession to the British throne dates back more than 1,000 years. In that long history, countless wars, battles, coups, and conquests have seen the throne change hands between rivaling parties on numerous occasions. But on at least one of those, in the winter of 1120 CE, the entire English line of succession was altered forever by a series of events a lot more peculiar than any of those.

At that time, England was still under the control of the Norman rulers of northern France. The Norman Conquest of 1066 had seen William, the Duke of Normandy, defeat England’s King Harold II at the Battle of Hastings. In doing so, William had taken control of Harold’s kingdom as the newly-crowned King William I of England.

The entire Norman kingdom was now divided between the northern half of France and the southern half of Great Britain. And lying between the two of them was the English Channel.

That fractured setup meant that the English kings of the Norman era were often obliged to divide their time between the French and English halves of their kingdom, and therefore were compelled cross the English Channel countless times a year. So, on November 25, 1120, a vessel called La Blanche-Nef (literally the “White Ship”) was chartered to carry the present English king, Henry I, the fourth surviving son of William the Conqueror, back across the Channel from France to England, along with all of the closest members of his court, much of his family, and his royal retinue.

At the last moment, however, Henry’s travel plans were changed. A separate vessel was chartered for the King to travel on alone, while much of the rest of his family and his court were left to travel aboard the White Ship. Precisely what happened next is unclear, but as the afternoon drifted into the evening, it seems likely that the aggravating delay to the White Ship’s departure caused by the King’s last-minute change of plans, or, perhaps, the freedom that came from no longer traveling under the King’s watchful gaze, led the 300 or so passengers on board the White Ship to organize an impromptu party. Casks of wine and ale were ordered and brought onto the ship in huge quantities so that, by the time the White Ship was finally ready to depart that night, most of its passengers and crew were by now roaring drunk.

Amid all of this revelry, the White Ship’s captain, Thomas Fitz-Stephen, was challenged by some of his passengers to race the King’s ship back to England. Knowing that his ship was indeed faster than one on which the King was now traveling, Fitz-Stephen accepted the challenge and set sail from the Norman port of Barfleur as quickly as possible. Within minutes of its departure, however, the White Ship came into trouble.

In the darkness of the winter night, the White Ship struck a submerged rock on her port side and quickly began to take on water. Just off the coast of Barfleur, the vessel suddenly capsized, throwing all of its passengers and crew into the freezing cold sea. The King’s son and immediate heir, Prince William Adelin, was hauled from the water by unknown rescuers and lifted onto a small skiff (essentially a lifeboat). The skiff began to head back to the shore, ferrying the next in line to the throne to safety—but when William heard his sister, Princess Mathilde, crying out in the darkness, he selflessly ordered the skiff to turn back towards the wreck, in an attempt to save her.

There were so many desperate people clinging to the wreck of the White Ship, however, that William’s lifeboat was soon swamped and dragged beneath the waves. So as the White Ship went down, so too did its passengers’ one and only chance of rescue; both Prince William and Princess Mathilde were never seen again. Indeed, according to some reports from the time, there was just one survivor of the White Ship disaster: a butcher from Rouen, recorded only as M Berold, who managed to cling all night to a shattered piece of the ship’s mast, while a thick fur overcoat he was wearing shielded him from the winter cold. He was found just after daybreak the following morning by a boat of local fishermen and taken back to the shore to recover.

Back in England, when news of the disaster reached King Henry, he was distraught. Almost all of his court, his closest friends and advisors, and his immediate family, including his heir, had all perished on board the White Ship. Ultimately, he was forced to name his only surviving legitimate child, his second daughter, Matilda, as his rightful successor. But the King’s choice was to prove an unpopular one.

Despite originally swearing an oath to the King to accept Matilda’s claim to the throne, on Henry’s death in 1135, the Barons of England promptly reneged on their agreement. England had never before been ruled by a queen alone, and, what’s more, Matilda’s husband Geoffrey V of Anjou, the founder of the Plantagenet dynasty, was considered by the Barons to be an enemy to England.

Instead, the Barons supported a rival claim to the throne by Stephen of Blois, the Count of Boulogne, who was one of Henry I’s nephews and a cousin of Matilda. For the next 18 years, the pair of rivals clashed repeatedly in a brutal fight to secure the throne, until Stephen proved victorious. He ruled as King of England until his death in 1154.

The rivalry between Stephen and Matilda (whom many consider the rightful heir) altered the course of the English line of succession. But, according to one account of the White Ship disaster, the entire affair only took place thanks to the most bizarre of circumstances back on that fateful night.

As one of the King’s nephews, it later emerged that Stephen too had been due to travel to England aboard the White Ship alongside many of the other members of his family. As the drinks were served to the waiting passengers, however, Stephen quickly fell ill (possibly due to imbibing a spoiled cask of ale) and was forced to leave the ship with a debilitating bout of diarrhea. His upset stomach, however, was to prove his salvation. Stephen left the ship, escaped the entire White Ship disaster, and survived to launch his claim to the throne 15 years later. The course of English history and England’s line of succession, ultimately, was altered beyond measure by nothing more than a queasy bout of seasickness!