Franklin's lost expedition was a failed British voyage of Arctic exploration led by Captain Sir John Franklin that departed England in 1845 aboard two ships, HMS Erebus and HMS Terror, and was assigned to traverse the last unnavigated sections of the Northwest Passage in the Canadian Arctic and to record magnetic data to help determine whether a better understanding could aid navigation. The expedition met with disaster after both ships and their crews, a total of 129 officers and men, became icebound in Victoria Strait near King William Island in what is today the Canadian territory of Nunavut. After being icebound for more than a year Erebus and Terror were abandoned in April 1848, by which point Franklin and nearly two dozen others had died. The survivors, now led by Franklin's second-in-command, Francis Crozier, and Erebus's captain, James Fitzjames, set out for the Canadian mainland and disappeared having presumably perished.Pressed by Franklin's wife, Jane, and others, the Admiralty launched a search for the missing expedition in 1848. In the many subsequent searches in the decades afterwards several relics from the expedition were uncovered, including the remains of two men that were returned to Britain. A series of scientific studies in modern times suggested that the men of the expedition did not all die quickly. Hypothermia, starvation, lead poisoning or zinc deficiency, and diseases including scurvy, along with general exposure to a hostile environment while lacking adequate clothing and nutrition, killed everyone on the expedition in the years after it was last sighted by Europeans in 1845. Cut marks on some of the bones recovered during these studies also supported allegations of cannibalism reported by Franklin searcher John Rae in 1854.

Despite the expeditions infamous status, it did explore the vicinity of what was ultimately one of many Northwest Passages to be discovered. Robert McClure led one of many expeditions to investigate the fate of Franklins expedition, and finally identified an ice-bound route connecting the Atlantic Ocean to the Pacific Ocean, returning alive. This trip too was beset by immense challenges and controversies. The Northwest Passage was not navigated by boat until 1906, when Roald Amundsen traversed the passage on the Gja.

In 2014 a Canadian search team led by Parks Canada located the wreck of Erebus in the eastern portion of Queen Maud Gulf. Two years later, the Arctic Research Foundation found the wreck of Terror south of King William Island, in the coincidentally named Terror Bay. Research and dive expeditions are an annual occurrence at the wreck sites, now protected as a combined National Historic Site.

Sir John Franklin (16 April 1786 – 11 June 1847) was a British Royal Navy officer and Arctic explorer. After serving in wars against Napoleonic France and the United States, he led two expeditions into the Canadian Arctic and through the islands of the Arctic Archipelago, in 1819 and 1825, and served as Lieutenant-Governor of Van Diemen's Land from 1839 to 1843. During his third and final expedition, an attempt to traverse the Northwest Passage in 1845, Franklin's ships became icebound off King William Island in what is now Nunavut, where he died in June 1847. The icebound ships were abandoned ten months later and the entire crew died, from causes such as starvation, hypothermia, and scurvy.