Believing that he had accounted for all of the defensive measures, O'Byrne took Curie to a depth of 65 feet (20 m) early on 20 December and, attempting an incursion into the harbor, heard the sounds of chains and wires dragging on the submarine's hull. When the sounds stopped after half a minute, O'Byrne brought Curie up to periscope depth to discover that he had only penetrated the outer net.[1] Curie's forward momentum carried her into the second net where she became "inextricably entangled".[7] When the submarine, still trapped in the net, was forced to surface for fresh air, Curie came under fire from the Austro-Hungarian destroyer Magnet and torpedo boat Tb 63 T which quickly sank her. Three of the twenty-six men on board were killed in the attack; the survivors — who included Curie's commander, O'Byrne — were all taken prisoner.[8][Note 1]