Mawson joined Ernest Shackleton's Nimrod Expedition (1907-1909) to the Antarctic, originally intending to stay for the duration of the ship's presence in the first summer. Instead both he and his mentor, Edgeworth David, stayed an extra year. In doing so they became, in the company of Alistair Mackay, the first to climb the summit of Mount Erebus and to trek to the South Magnetic Pole, which at that time was over land.