For over a century, climbing Mount Everest was a great challenge. Early attempts at reaching the top of the mountain brought climbers to about 28,000 feet. But the mountain was just too high. The physical effects of high altitudes and the limitations of available equipment made attempts to reach the top impossible. In 1924, British climbers Andrew "Sandy" Irvine and George Mallory climbed past 28,000 feet. They were once seen nearing the top of the mountain but disappeared soon after. Whether or not Mallory and Irvine reached the top remains a mystery. Since their climb, many expeditions have gathered clues and tried to determine for certain whether the two reached the top, but none have succeeded so far. The top of Mount Everest was finally reached by Edmund Hillary and Tenzing Norgay in 1953.