Oct. 6 (UPI) -- Sir Ernest Shackleton's polar ship, Endurance , was considered to be the strongest of its time but the vessel had significant structural weaknesses that caused its demise -- and an analysis now suggests the famed explorer knew it when he set out for the Antarctic. A weakness in the ship's rudder has long been called the fatal flaw in Endurance 's design that caused the ship to be crushed by sea ice and sink to the bottom of the Weddell Sea in late 1915. Researchers at Aalto