Southwest of the summit, the icefall is regarded as one of the most dangerous stages of the South Col route to Everes The Khumbu Glacier forming the icefall moves an estimated 0.9 to 1.2 m (3 to 4 ft) down the mountain every day, a speed that opens large crevasses with little warning and suddenly collapses large towers of ice (called seracs), ranging in size that of cars to large houses, that tumb
le down the glacier from time to time. Most climbers try to cross before sunrise when the icefall, still partially frozen from nighttime cold, is less able to move. As intense sunlight warms the area, friction within the ice structure declines, which increases the rate of crevasses opening and blocks of snow and ice falling. The most dangerous time to cross is generally mid- to late-afternoon. Strong, acclimatized climbers can ascend the icefall in a few hours, while climbers who go through it for the first time, or lack acclimatization or experience, tend to complete the journey in 10–12 hours. "Camp I" on Everest's South Col route is typically slightly beyond the top of the Khumbu Icefall. On occasion, a large block of ice crashes down in the vicinity of a climber, producing a blast of displaced air and snow that can result in a "dusting" from a billowing cloud of light ice and snow—a very unnerving experience. A climber caught in an avalanche or other "movement" event in the icefall can do very little because it is not possible to run away or even to know which direction to run; therefore, climbers train for entrapment by heavy blocks of ice as well as rescuing others.
