 Complete the following phrase:  During the Nimrod Expedition led by Ernest Shackleton in 1907, parties led by Edgeworth David became the first to climb Mount Erebus and to reach the South Magnetic Pole. Douglas Mawson, who assumed the leadership of the Magnetic Pole party on their perilous return, went on to lead several expeditions until retiring in 1931. In addition, Shackleton and three other members of his expedition made several firsts in December 1908  February 1909: they were the first humans to traverse the Ross Ice Shelf, the first to traverse the Transantarctic Mountains via the Beardmore Glacier, and the first to set foot on the South Polar Plateau. 