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Dummy mine

Plastic drums filled with sand or concrete are periodically rolled off the side of ships as real mines are laid in large mine-fields. These inexpensive false targets (designed to be of a similar shape and size as genuine mines) are intended to slow down the process of mine clearance: a mine-hunter is forced to investigate each suspicious sonar contact on the sea bed, whether it is real or not. Often a maker of naval mines will provide both training and dummy versions of their mines.