Passage: With the loss of the ship the transcontinental plans were abandoned, and the focus shifted to that of survival. Shackleton's intention now was to march the crew westward, to one or other of several possible destinations. His first thought was for Paulet Island, where he knew there was a hut containing a substantial food depot, because he had ordered it 12 years earlier while organising relief for Otto Nordenskjöld's stranded Swedish expedition. Other possibilities were Snow Hill Island, which had been Nordenskiöld's winter quarters and which was believed to contain a stock of emergency supplies, or Robertson Island. Shackleton believed that from one of these islands they would be able to reach and cross Graham Land, and get to the whaling outposts in Wilhelmina Bay. He calculated that on the day Endurance  was abandoned they were 346 miles (557 km) from Paulet Island. Worsley calculated the distance to Snow Hill Island to be 312 miles (500 km), with a further 120 miles (190 km) to Wilhelmina Bay. He believed the march was too risky; they should wait until the ice carried them to open water, and then escape in the boats. Shackleton over-ruled him.Before the march could begin, Shackleton ordered the weakest animals to be shot, including the carpenter Harry McNish's cat, Mrs Chippy, and a pup which had become a pet of the surgeon Macklin. The company set out on 30 October 1915, with two of the ship's lifeboats carried on sledges. Problems quickly arose, as the condition of the sea ice around them worsened. According to Hurley the surface became "a labyrinth of hummocks and ridges", in which barely a square yard was smooth. In three days, the party managed to travel barely two miles (3.2 km), and on 1 November, Shackleton abandoned the march; they would make camp and await the break-up of the ice. They gave the name "Ocean Camp" to the flat and solid-looking floe on which their aborted march had ended, and settled down to wait. Parties continued to revisit the Endurance wreck, which was still drifting with the...

Question: What date did the ship finally sink below the ice?
21 November