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.

HMS Terror was a specialised warship and a newly developed bomb vessel constructed for the Royal Navy in 1813. She participated in several battles of the War of 1812, including the Battle of Baltimore with the bombardment of Fort McHenry. She was converted into a polar exploration ship two decades later, and participated in George Back's Arctic expedition of 1836–1837, the successful Ross expedition to the Antarctic of 1839 to 1843, and Sir John Franklin's ill-fated attempt to force the Northwest Passage in 1845, during which she was lost with all hands along with HMS Erebus.