Mount Everest: 1921 expedition George Mallory (seated, far left) and Guy Bullock (seated, third from the left), planners of the reconnaissance expedition to Mount Everest in 1921. The other mountaineers shown are (clockwise from top left) A.F.R. Wollaston, Charles Howard-Bury, Alexander Heron, Harold Raeburn, Henry T. Morshead, and Oliver Wheeler. The Granger Collection, New York

George Mallory was a schoolmaster in England and a seasoned mountain climber. He had trained on the most-difficult routes up the Alps before being recruited for the first major climbing expedition up Mount Everest in 1921. That first attempt was thwarted by high winds, and a second attempt in 1922—which also failed—involved an avalanche and the death of seven porters. Finally in 1924 he set out on a third expedition. Mallory and another climber, Andrew Irvine, went off to attempt the summit on June 8 and were never seen again. They left the world to wonder what had taken place on that fateful day, including whether before disappearing they had become the first climbers ever to reach the summit. Irvine’s axe was found in 1933 at about 27,750 feet (8,460 meters), which seemed to indicate that they did not make it to the top and likely fell to their deaths. Seventy-five years later, in 1999, an expedition discovered Mallory’s body at 26,760 feet (8,160 meters). As of 2015, Irvine’s remains had not been found, and the exact circumstances of their deaths are as yet undetermined.