evidence: Of the 14 mountains that rise at least 8,000 meters (26,246 feet), K2 remains the only peak unclimbed during winter. [...] "The Sherpas are much stronger than many more famous climbers," Txikon says wryly. This group joined Txikon on two consecutive, ultimately unsuccessful winter trips to Everest over the last two years, so they have a lot of experience together climbing in these extreme conditions. Having been shut down by Everest in winter twice, however, the team wanted to mix it up and try a new objective to keep the motivation high. Despite it being a far tougher mountain, perhaps they'd have better luck on K2, they reasoned. [...] K2 has long been considered too tough even to consider in winter. In fact, a winter ascent has only been attempted five times prior to this year. "It’s the worst place on Earth," Bielecki says. No team has even made it past 25,100 feet—and with the summit standing at 28,251 feet, those last 3,000 feet are going to be the toughest.