(Fortune) -- It's 25 below outside, and the heat in the van is busted. Randy Boyer, a burly ConocoPhillips contractor in thermal coveralls, navigates the slick ice road. "This is nothing," he says, keeping his eye on the thin red line running down the center of the road. "The other week we had a whiteout, and I was stuck in my truck for 36 hours." Right now we're some 250 miles north of the Arctic Circle, and it's so white outside that the distant horizon appears to blend seamlessly into the blustery sky.

Boyer pulls over into a small mobile base camp to switch trucks. We're deep in the tundra of Alaska's North Slope, and you need a vehicle with tanklike treads or special tires to venture off-road. After a bone-rattling 15-minute ride - the tundra only looks flat - we catch up with a red truck creeping across the frozen landscape. A worker bundled in a blue coat of Michelin-man dimensions trudges behind it, uncoiling orange cable that looks like the extension cord you might keep in your garage. Every 100 feet or so, he plants three small blue geophones into the ground.

He's setting up to "shoot seismic," an oil driller's term for a laborious surveying process that maps the earth's subsurface. It will take a team of 100 workers more than a week to lay a precise grid of 21,000 geophones, which act as motion detectors, along 145 miles of wire. After that a truck known as a "vibrator" will make stops along the grid, lowering a large plate onto the ground. The apparatus creates seismic waves - tiny earthquakes - that are recorded by the geophones. Geologists in Anchorage will then spend six months transforming that data into a picture of the underground formations. Some of them, they hope, will contain oil.

The folks at Conoco surveyed this slice of barren land about a decade ago. But times are a bit desperate up here in North America's largest oil region, and they've come back. "We're looking to see if we left anything behind," says Jim Darnall, an acquisition geophysicist for ConocoPhillips, as he brushes ice off his bushy gray beard. "We're trying to milk this field anyway we can."

Is this what America's late-20th-century oil paradise has been reduced to - the petroleum equivalent of rooting for loose change in the cushions of a sofa? U.S. crude production is at its lowest since 1949, and nowhere has that decline been steeper than in Alaska, where oil output is less than half what it was a decade ago. The fields that since the late 1970s have provided more than 20% of America's oil are slowly running dry. It's a phenomenon that is hardly limited to Alaska. The world's five largest oil companies are replacing only 82% of the oil they pump each year, as once-prodigious fields fade and state entities in such countries as Venezuela and Russia consolidate ever more control over their oil and gas.

The combination of falling reserves and $100-plus oil is sparking a frenzy of oil and gas activity in Alaska the likes of which hasn't been seen since the state's initial oil boom more than three decades ago. ConocoPhillips (COP, Fortune 500), Alaska's biggest producer and America's third-largest oil company, is spending huge sums to re-explore old stomping grounds like the North Slope. The company is also investing in heavy-oil technology and early preparation for a proposed $30 billion natural gas pipeline. "We think the Arctic is the new frontier," says Conoco CEO Jim Mulva, "and it's not just in Alaska. The potential exploration opportunities go all the way around the Arctic Circle."

The excitement extends even farther north, where the shrinking ice cap is helping spur a new race for territorial supremacy. In August, Russia planted a flag 2 1/2 miles below sea level at the actual North Pole, laying claim to what it says are vast quantities of oil and gas. Some experts estimate that a quarter of the planet's undiscovered energy resources are buried at the top of the planet. But that figure is highly speculative - much of the Arctic is unexplored - and even if you find something, how do you transport it through a sea of ice?