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From BoardGameGeek:

A unique variation on the format of Battleship. The game is different in having a three dimensional board. Players hide their submarines and a mine on the three underwater levels on one side, and place their surface ships on the top surface level of the other side. Players move a ship to a desired grid location and then fire a depth charge, marking off hits, near misses, and misses on their opponent's submarines. Submarines can fire a torpedo at surface vessels that are in the same column. Play alternates back and forth until one player sinks all three of his opponent's subs or all of his opponent's surface ships (almost never happens).