The way I am picturing it is that of hooks on a chain lying just below the surface that can be rushed back towards a crane, pulling any ship who crosses the lin

The way I am picturing it is that of hooks on a chain lying just below the surface that can be rushed back towards a crane, pulling any ship who crosses the line. Looking at a map, the placement of such a line is obvious: https://en.wikipedia.org/wiki/Sicilian_Expedition#/media/File:Athenian_Siege_of_Syracuse.png https://en.wikipedia.org/wiki/Sicilian_Expedition#/media/Fil...