Complex sentence: In October 1915, "Endurance" had been sunk by the pack ice in the Weddell Sea, leaving Sir Ernest Shackleton and his companions adrift on a precarious ice surface.
Split into simpler sentences: In October 1915, "Endurance" had been sunk by the pack ice in the Weddell Sea. The event had left Sir Ernest Shackleton and his companions adrift on a precarious ice surface.