As ships made their way through the Arctic on adventure and whaling missions, explorers pieced together knowledge of the Northwest Passage.

Gilbert Pajot/The Bridgeman Art Library/Getty Images

Western explorers began looking for a shortcut between Europe and Asia more than 500 years ago. Some went so­uth, where they discovered the enticingly narrow Panamanian isthmus. Others aimed north, bumping into the American continent and pursuing inland waterways. Henry Hudson found himself trapped with a mutinous crew in a bay that lead nowhere near as close to the Pacific as he had hoped it would. Captain James Cook concluded that there was no passage through North America after making it to the Bering Straight; he was stabbed in the back by Hawaiian natives on his return home. But after centuries of expeditions by sea and land, explorers eventually pieced together knowledge of a viable -- though frozen -- route from Baffin Bay to the Bering Sea, and thus from the Atlantic to the Pacific. The fabled Northwest Passage did exist after all.­

This pas­sage runs 900 miles (1,450 km) through a chain of deep Arctic channels and what are now Canada's Arctic Islands. Since the entrance is 500 miles (800 km) north of the Arctic Circle, access to the passage requires dangerous navigation around bobbing icebergs [source: Encyclopaedia Britannica]. And although the passage was traversed by sled and ship in 1854, completed by ship alone over a period of three years in 1906, and finally navigated in a single season by 1944, it hasn't been considered "fully navigable" until now [source: ESA].

­That's because in 2007, the European Space Agency (ESA) released a mosaic of satellite images showing the Arctic sea passage m­elted and clear. The planet's rising temperatures have caused Arctic ice to thaw at a rate faster than normal. Sea ice is bright white and reflective, allowing most of the sun's rays bounce off of its surface. But when sea ice melts, it exposes dark ocean waters. Since the water's surface is dark instead of light, it absorbs solar energy instead of reflecting it. As the ocean warms, new ice has trouble forming. In this way, the ice melt of a summer is compounded over the next summer, and the next. In the past decade, Arctic ice has melted at about 38,610 sq miles (100,000 sq km) per year. But between 2006 and 2007, it dropped 386,102 sq miles (1 million sq km) -- the biggest drop since measurements began in 1978 [source: ESA]. With such a sharp decline in ice cover, the passage thawed again in 2008.

The drop in ice cover is unsettling environmentally, but it's stirred up the interesting prospect of a new shipping artery to rival the Suez and Panama Canals.