Naval mine

Naval Mines were explosive weapons used by the Continental Army during the American Revolutionary War when facing the overwhelming naval force of the British navy. Commonly Naval Mines were set up in harbour regions to act as a deterrent and impede land invasion by sea and the most common type during the 1700s was the floating keg mine.

The American Revolution
Naval mines originated in the sixteenth century, but their use in naval combat began in the American Revolutionary War by David Bushnell, who placed such devices under and near the hulls of British ships in New York harbor using a small one‐man, wooden submarine he invented.

Description
Underwater explosive devices such as Navel Mines are designed to sink ships or other seaborne craft or by such threat to prevent them from using an area. Their firing mechanisms were either the traditional pressure points which detonated the explosive on contact by the approach of a vessel.

Most mines were automatic, but some harbor mines, controlled via a pressure plate at its top, could be turned off to allow transit of friendly vessels. Moored mines are tethered to sinkers, and they float at predetermined depths generally to cut off particular areas. Traditionally they have been contact mines floating just below the water to damage surface ships that touch them.

Mines are small, relatively inexpensive, easily laid down, and require little maintenance. Yet they have the explosive ability to sink or badly damage even large vessels by blowing open their hull below the waterline. Consequently, smaller naval powers have often used them to impede the larger fleets of major powers.

Source

 * GamesCom 2012 Assassin’s Creed III Naval trailer.