It’s not the kind of ad likely to arouse much enthusiasm these days: "Men wanted: small wages, bitter cold, long periods of complete darkness, constant danger, safe return doubtful." When the Anglo-Irish explorer Ernest Shackleton posted it for his proposed expedition across the Antarctic continent in 1914, 5000 replied. The two dozen or so chosen got more than they bargained for. Dismissed by the likes of Winston Churchill as a "sterile quest," Shackleton’s dream ended a day’s sail from shore when his ship the Endurance froze forever in pack ice. What followed was a different kind of discovery, that of the limits of human endurance, doggedly recorded in this documentary from George Butler — an expansion of his 40-minute IMAX film — that’s based on the book by Caroline Alexander and narrated by the muscular voice of Liam Neeson. Shackleton had the foresight to include a cameraman and an artist on board, and their movie footage, stills, paintings, and drawings highlight this Sisyphean ordeal in images that radiate with the beauty of the margins of existence. Unfortunately, the film proves marginal on such matters as the historical context — while the crew of the Endurance teetered for two years on the brink of extinction, Europe immolated itself in the trenches. Neither does it plumb the nature of the charismatic, enigmatic Shackleton, whose final 36-hour trek across the unnamed mountains of South Georgia Island inspired a passage in T.S. Eliot’s The Waste Land.