Complex sentence: While Worsley had been retrieving McNish and others from King Haakon Bay, Shackleton was advised of the fate of his Ross Sea party which had been tasked with laying depots on Shackleton's intended route across Antarctica.
Split into simpler sentences: While Worsley had been retrieving McNish and others from King Haakon Bay, Shackleton had become aware of the fate of his Ross Sea party. This had been tasked with laying depots on Shackleton's intended route across Antarctica.