Why is the Arctic changing?
Global warming is melting the ice sheet covering the top of the world. The amount of ice during the summer months has declined more than 40 percent since the 1970s, a trend that most climate scientists say is a consequence of man-made greenhouse gas emissions. By as early as 2030, some researchers believe, the Arctic could have ice-free summers. "Climate change is actually doing what our worst fears dictated," said Jennifer Francis, a sea-ice expert at Rutgers University. But for countries that border the Arctic, the melt offers big benefits. New shipping lanes between Europe and the Pacific are opening up. Vast amounts of oil and natural gas that were once locked beneath the ice can now be exploited.

How much oil and gas is there?
The Arctic Circle covers about 6 percent of the earth's surface, but could hold as much as 20 percent of the world's undiscovered hydrocarbon resources, according to the U.S. Geological Survey. That includes 90 billion barrels of oil, or about as much oil as the entire U.S. uses over 12 years, and even more natural gas — about a third of the world's untapped reserves. As the ice has retreated, energy companies have moved in, even though it's not yet clear which nations own these resources. Royal Dutch Shell has spent more than $4.5 billion searching for oil off Alaska's coast since 2005, and U.S. giant ExxonMobil has signed an exploration deal with Russia, as have Britain's BP, Norway's Statoil, and Italy's Eni. Not everyone in the industry supports this rush north. Christophe de Margerie, chief executive of French oil giant Total SA, warns that the risks of drilling in this extreme environment are too high, and that a spill in this pristine wilderness would be a "disaster."

Why is it so hard to drill there?
In summer months, gale force winds move giant floes of ice that can tear through the sides of oil rigs or rip up undersea pipelines. And in winter, when daylight is minimal and temperatures drop as low as minus 50, much of the region becomes covered with sea ice and is inaccessible. Amid such hazards, said Simon Boxall, a British oil-spill expert, "It is inevitable you will get a spill." Capping broken wells in winter would be difficult if not impossible, and cleanups in the harsh conditions would pose unprecedented challenges.

What if there's a leak?
Shell has developed a 20-foot-tall steel "containment dome," which can be lowered onto a leaking wellhead and funnel oil to a tanker on the surface. But in a sea test last year, the dome malfunctioned and was "crushed like a beer can" by water pressure, according to government documents. Industry officials argue that the Arctic's freezing waters could actually help with a cleanup, as oil breaks down slowly in cold temperatures, giving more time to skim the spill from the surface. But environmentalists say that during the 2010 spill in the calm waters of the Gulf of Mexico, the industry managed to skim only 3 percent and burn 5 percent of the spilled oil. Much of the remainder sank to the ocean floor, where it was digested by bacteria. Those bacteria aren't as plentiful in the Arctic. A Shell official says that in the Arctic, "it would be rather ridiculous of us to make any kind of performance guarantee."

Who owns the Arctic?
That's very unclear. Under the 1982 United Nations Convention on the Law of the Sea, coastal nations have exclusive rights to underwater resources up to 200 miles from their own shore. They can extend that claim by proving the seabed in a particular area is an extension of their continental shelf. Russia, Denmark, and Canada are all seeking to claim ownership of the Lomonosov Ridge — a 1,200-mile underwater mountain range that runs through the Arctic Ocean. The U.S. has been left out of this jockeying since the Senate has refused to ratify the treaty, fearing it would cede American sovereignty to the U.N. "Until we ratify that treaty, we will remain on the sidelines [in the Arctic]," said Heather Conley, a fellow at Washington's Center for Strategic & International Studies. In case boundary disputes cannot be settled peacefully, Canada and Russia are preparing to defend their claims militarily.

What steps are they taking?
Canada is investing $3 billion in a fleet of offshore patrol ships, and is developing unmanned drone aircraft that can cope with the region's freezing temperatures. Russia, meanwhile, is restoring a Soviet-era base off Siberia and recently sent a warning to foreigners not to interfere with its Arctic interests by jailing the 30-member crew of a Greenpeace ship who protested at an offshore platform. Both countries have also staged military exercises at their northern borders, with the Russians recently sending 10 warships to patrol Arctic waters. "The 21st century will see a fight for resources," said Dmitry Rogozin, Russia's former ambassador to NATO. "Russia should not be defeated in this fight."

Opening the Northwest Passage
For centuries, traders have dreamed about sailing ships through the ice-choked Northwest Passage, a shortcut from the Atlantic to the Pacific that runs across Canada's north coast. With ice cover shrinking due to global warming, that fabled sea lane is now open for business. Last September, the Nordic Orion — an ice-strengthened carrier hauling coal from Vancouver to Finland — became the first major commercial vessel to make the voyage, cutting 1,000 miles off its normal route through the Panama Canal and saving $80,000 in fuel. Still, the risks of traveling the Northwest Passage will remain substantial for the foreseeable future. Michael Byers, author of International Law and the Arctic, notes that small, hard-to-spot icebergs have been known to puncture even fortified hulls. "Then there is 'icing,'" he says, "which occurs when ocean spray freezes onto the superstructure of a ship, causing it to become top-heavy and capsize." The Panama Canal, he says, will not lose many customers.