Forty years ago, on May 11, 1986, Jean-Louis Étienne reached the geographic North Pole alone, after 63 days of walking and more than a thousand kilometers across the pack ice. Hauling his sled, pushing forward no matter what, through pressure ridges shaped by the wind, extreme cold — minus 52 degrees Celsius! — and the drift of the frozen Arctic Ocean. And then, at the end of this “inner pole” —...