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 two dozen men, including Franklin, 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, presumably having 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 artefacts from the expedition were discovered, including the remains of two men, which 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 a whaling ship in July 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 expedition's notorious failure, it did succeed in exploring the vicinity of one of the many Northwest Passages that would eventually be discovered. Robert McClure led one of the expeditions that investigated the fate of Franklin's expedition, a voyage which was also beset by great challenges and later controversies. McClure's expedition returned after finding an ice-bound route that connected the Atlantic Ocean to the Pacific Ocean. The Northwest Passage was not navigated by boat until 1906, when Roald Amundsen traversed the passage on the Gjøa.

In 2014, a 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 body of water named Terror Bay. Research and dive expeditions are an annual occurrence at the wreck sites, now protected as a combined National Historic Site called the Wrecks of HMS Erebus and HMS Terror National Historic Site.

The search by Europeans for a western shortcut by sea from Europe to Asia began with the voyages of Portuguese and Spanish explorers such as Bartolomeu Dias, Vasco da Gama and Christopher Columbus in the 15th century. By the mid-19th century numerous exploratory expeditions had been mounted. These voyages, when successful, added to the sum of European geographic knowledge about the Western Hemisphere, particularly North America. As that knowledge grew, exploration gradually shifted towards the Arctic.^{[citation needed]}

Sixteenth- and seventeenth-century voyagers who made geographic discoveries about North America included Martin Frobisher, John Davis, Henry Hudson and William Baffin. In 1670 the incorporation of the Hudson's Bay Company (HBC) led to further exploration of the Canadian coastlines, interior and adjacent Arctic seas. In the 18th century explorers of this region included James Knight, Christopher Middleton, Samuel Hearne, James Cook, Alexander MacKenzie and George Vancouver. By 1800 their discoveries had conclusively demonstrated that no Northwest Passage between the Pacific and Atlantic oceans existed in the temperate latitudes.

In 1804 Sir John Barrow became Second Secretary of the Admiralty, a post he held until 1845. Barrow began pushing for the Royal Navy to find a Northwest Passage over the top of Canada and to navigate toward the North Pole, organising a major series of expeditions. Over those four decades explorers including John Ross; David Buchan; William Edward Parry; Frederick William Beechey; James Clark Ross (nephew of John Ross); George Back; Peter Warren Dease and Thomas Simpson led productive expeditions to the Canadian Arctic. Among those explorers was John Franklin, who first travelled to the region in 1818 as second-in-command of an expedition towards the North Pole on the ships Dorothea and Trent. Franklin was subsequently leader of two overland expeditions to and along the Canadian Arctic coast, in 1819–1822 and 1825–1827.

By 1845 the combined discoveries of all these expeditions had reduced the unknown parts of the Canadian Arctic that might contain a Northwest Passage to a quadrilateral area of about 181,300 km² (70,000 sq mi). It was in this unexplored area that the next expedition was to sail, heading west through Lancaster Sound, then west and south – however ice, land and other obstacles might allow – with the goal of finding a Northwest Passage. The distance to be navigated was roughly 1,670 km (1,040 mi).