A naval mine is a self-contained explosive device placed in water to destroy ships or submarines. Unlike depth charges, mines are deposited and left to wait until they are triggered by the approach of or contact with an enemy ship. Naval mines can be used offensively, to hamper enemy ships or lock them into a harbour; or defensively, to protect friendly ships and create "safe" zones.
Mines can be laid in many ways: by purpose-built minelayers, refitted ships, submarines, or aircraft-and even by dropping them into a harbour by hand. They can be inexpensive: some variants can cost as little as a thousand United States dollars (USD), although more sophisticated mines can cost millions of USD, be equipped with several kinds of sensors, and deliver a warhead by rocket or torpedo.
Their flexibility and cost-effectiveness make mines attractive weapons to the less powerful belligerent in asymmetric warfare. The cost of producing and laying a mine is usually anywhere from 0.5% to 10% of the cost of removing it, and it can take up to 200 times as long to clear a minefield as to lay it. Parts of some World War II naval minefields still exist, because they are too extensive and expensive to clear; some of these mines might remain dangerous for hundreds of years.
There are three main uses of mines: offensive, defensive, and psychological. Offensive mines are placed in enemy waters, outside harbours and in important shipping routes to sink civilian and military ships. Defensive minefields protect a coast from enemy ships and submarines and force them into areas that are easier to defend.
Minefields designed for psychological effect are usually placed in trade routes and are used to stop shipping to an enemy nation. They are also spread out thinly, to create a feeling of random minefields in large areas. A single mine along a shipping route can stop shipping for days until the entire area is swept.
International law requires nations to declare when they mine an area, in order to make it easier for civil shipping to avoid the mines. The warnings do not have to be specific; during World War II, Britain declared simply that it had mined the English Channel, North Sea, and French coast.