Mine countermeasures

A naval mine is a self-contained explosive device placed in water to damage or destroy surface 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 vessel. Naval mines can be used offensively—to hamper enemy shipping movements or lock vessels into a harbour; or defensively—to protect friendly vessels 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 US$2000, though more sophisticated mines can cost millions of dollars, be equipped with several kinds of sensors, and deliver a warhead by rocket or torpedo.

Their flexibility and cost-effectiveness make mines attractive 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. It is possible for some of these 1940s-era mines to remain dangerous for many years to come.

Mines have been employed as offensive or defensive weapons in rivers, lakes, estuaries, seas, and oceans, but they can also be used as tools of psychological warfare. Offensive mines are placed in enemy waters, outside harbours and across important shipping routes with the aim of sinking both merchant and military vessels. Defensive minefields safeguard key stretches of coast from enemy ships and submarines, forcing them into more easily defended areas, or keeping them away from sensitive ones.

Minefields designed for psychological effect are usually placed on trade routes and are used to stop shipping from reaching an enemy nation. They are often spread thinly, to create an impression of minefields existing across large areas. A single mine inserted strategically on a shipping route can stop maritime movements for days while the entire area is swept.

...
Wikipedia