The quest for the Northwest Passage -- a navigable sea route through the Canadian arctic -- was long and dangerous. Even so, when Sir John Franklin led two ships, the Erebus and Terror, on a last mission to complete mapping of North America's icy archipelago, there was every reason for confidence.

The Terror and Erebus were last seen by European whalers off the coast of Greenland in the summer of 1845, after which they simply disappeared into the frozen north.