In northern Alaska’s Brooks Range, the earth as most of us know it comes to an end. From Fairbanks, the northernmost city on the North American road grid, drive up the graveled Dalton Highway. Unpeopled boreal forest stretches in all directions. About 200 miles on, you pass the arctic circle, beyond which the sun never sets in midsummer, nor rises in midwinter. Eventually, the trees thin out, and look scrawnier. The rolling landscape rises into big mountains, and you are threading through the bare, razor-edged peaks of the Brooks. Midway through the mountains, scattered spruces cling only to valley bottoms; further upslope is tundra, covered only with low-lying plants. At about 320 miles from Fairbanks, you pass the last little trees. Beyond lie the barren lands of the North Slope, ending at the industrial arctic-coast hamlet of Deadhorse and the oil fields of Prudhoe Bay—the only reason this road is here at all.