Complex sentence: Shackleton's original unpublished plan was to base himself at the old Discovery Expedition headquarters in McMurdo Sound and from there mount attempts on the geographical South Pole and the South Magnetic Pole.
Split into simpler sentences: Shackleton's original unpublished plan envisaged basing himself at the old Discovery Expedition headquarters in McMurdo Sound. From there he propsed to launch attempts to reach the geographical South Pole and the South Magnetic Pole.