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The summit of the world's second-highest mountain is within Alan Arnette's reach.

On Saturday, the Fort Collins mountaineer began the final push to the top of 28,251-foot-tall K2. Arnette and his fellow climbing partners with Madison Mountaineering left Camp 4 at 25,200 feet at about 11 p.m. Pakistan time. They could summit K2 Sunday afternoon.

If successful, Arnette would be only the 15th American to summit K2, considered more challenging and more dangerous than the world's tallest peak, Mount Everest. Sunday is Arnette's 58th birthday.

Extreme altitude, fickle and frigid weather and hard-packed snow and ice are just some of the challenges Arnette and his team faces during the 12-plus hour climb to the summit.

The major obstacle to the summit is the infamous narrow, 300-foot couloir known as the "Bottleneck." Slopes are as steep as 80 degrees, and above, a wall of seracs — large blocks of glacial ice — threaten to break at any moment. In 2008, the serac broke loose, raining huge chunks of ice on climbers, sweeping away their fixed ropes. The serac fall was one factor that led to the deadliest day on K2 when 11 climbers lost their lives on the mountain.

Read more at the Coloradoan: http://noconow.co/UupIhp

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