A defensive device deployed across a harbor entrance to prevent submarines gaining access. It was to avoid such a defense that the midget submarine eventually destroyed by Ward attempted to follow Antares on the morning of 7 December 1941. Earlier that morning the minesweepers Crossbill and Condorhad entered the harbor at 0458, having reported a possible periscope sighting just over an hour before. The net remained open until 0846 and it is possible that the midget submarine attacked by Monaghan entered at that time. Alternatively, as the net was only 45 feet deep and the channel went down to 72 feet, a submarine might have gone under the closed net as the overall height of a midget submarine was 20 feet.