Why did explorers spend four centuries hunting for the Northwest Passage?

For about four centuries, Europeans wanted a shortcut to Asia that went around the top of North America. Ever since Columbus in 1492, explorers had been looking for a northern sea route to Asia, and they were blocked by the continents, by ice or by rough water. They called the hoped-for route the Northwest Passage.

The Arctic was poorly understood. Some people believed seawater could not freeze, and explorers thought an open sea must lie near the North Pole. In 1845 Sir John Franklin left with two lavishly equipped ships, sure that less than 500 km of unexplored coast remained. His ships froze in near King William Island in 1846, Franklin died in 1847, and the ships were abandoned in 1848. The search for Franklin mapped much of the Arctic, and one search party, Robert McClure's, crossed it from west to east in 1850-54, partly by ship and partly by sledge.

The first person to sail the whole passage solely by ship was the Norwegian Roald Amundsen. He set out in 1903 with a crew of six in Gjøa, a converted herring boat, hugging the shore and living off the land because supporting big crews had contributed to the failure of Franklin's expedition. He was iced in for nearly two years, his crew learned from the local Inuit, and the voyage was completed in 1906. Gjøa's route was too shallow, in places only 3 ft deep, to be a shipping lane.

The ice itself has changed since then: on 21 August 2007 the passage opened to ships without an icebreaker, for the first time in records going back to 1972. That has reopened the old argument about who owns the water: Canada calls it internal waters, while the United States says it is an international strait.