Answer by taking a quote from the following article:

George Herbert Leigh Mallory (18 June 1886 - 8 or 9 June 1924) was an English mountaineer who took part in the first three British expeditions to Mount Everest, in the early 1920s. During the 1924 British Mount Everest expedition, Mallory and his climbing partner, Andrew "Sandy" Irvine, disappeared on the North-East ridge during their attempt to make the first ascent of the world's highest mountain. The pair were last seen when they were about 800 vertical feet (245 m) from the summit. Mallory's ultimate fate was unknown for 75 years, until his body was discovered on 1 May 1999 by an expedition that had set out to search for the climbers' remains.

Mallory participated in the initial 1921 British Reconnaissance Expedition, organised and financed by the Mount Everest Committee, that explored routes up to the North Col of Mount Everest. The expedition produced the first accurate maps of the region around the mountain, as Mallory, his climbing partner Guy Bullock and E. O. Wheeler of the Survey of India explored in depth several approaches to its peak. Under Mallory's leadership, and with the assistance of around a dozen Sherpas, the group climbed several lower peaks near Everest. His party were almost certainly the first Westerners to view the Western Cwm at the foot of the Lhotse face, as well as charting the course of the Rongbuk Glacier up to the base of the North Face. After circling the mountain from the south side, his party finally discovered the East Rongbuk Glacier--the highway to the summit now used by nearly all climbers on the Tibetan side of the mountain. By climbing up to the saddle of the North Ridge (the 23,030 ft (7,020 m) North Col), they spied a route to the summit via the North-East Ridge over the obstacle of the Second Step. In 1922 Mallory returned to the Himalayas as part of the party led by Brigadier-General Charles Bruce and climbing leader Edward Strutt, with a view to making a serious attempt on the summit. Eschewing their bottled oxygen, which was at the time seen as going against the spirit of mountaineering, Mallory, along with Howard Somervell and Edward Norton almost reached the crest of the North-East Ridge. Despite being hampered and slowed by the thin air, they achieved a record altitude of 26,980 ft (8,225 m) before weather conditions and the late hour forced them to retreat. A second party led by George Finch reached an elevation of approximately 27,300 ft (8,321 m) using bottled oxygen both for climbing and--a first--for sleeping. The party climbed at record speeds, a fact that Mallory seized upon during the next expedition. Mallory organised a third unsuccessful attempt on the summit, departing as the monsoon season arrived. While Mallory was leading a group of porters down the lower slopes of the North Col of Everest in fresh, waist-deep snow, an avalanche swept over the group, killing seven Sherpas. The attempt was immediately abandoned, and Mallory was subsequently accused of poor judgement, including by expedition participants such as Dr. Longstaff. Mallory is famously quoted as having replied to the question "Why did you want to climb Mount Everest?" with the retort "Because it's there", which has been called "the most famous three words in mountaineering". There have been questions over the authenticity of the quote, and whether Mallory actually said it. Some have suggested that it was a paraphrase by a newspaper reporter, but scrutiny of the original report in The New York Times leaves this unresolved. The phrase was certainly consistent with the direct quotes cited in The New York Times report, so it appears not to misrepresent Mallory's attitude. CANNOTANSWER

What else happened at Rongbuk Glacier?

his party finally discovered the East Rongbuk Glacier--the highway to the summit now used by nearly all climbers on the Tibetan side of the mountain.