How Do Sea Mines Work? Contact, Influence and Limpet Mines Explained

A sea mine is a sealed case holding an explosive charge and a firing mechanism, placed in the water to sink or cripple ships that come near it.

Mines differ by where they sit. A moored mine is buoyant and held just below the surface by a cable fixed to an anchor on the seabed. A bottom mine is heavier and lies on the seabed itself, so it works only in fairly shallow water, and much of the Gulf is shallow enough. A drifting mine floats free with the current. A limpet mine is a smaller charge fixed to a ship’s hull by hand, usually by divers or from a small boat. Iran is also reported to hold a rocket mine, the Chinese-designed EM-52, which waits on the seabed and fires a warhead upward at a ship passing above.

Mines are cheap, and the damage they do is not. The Iranian mine that holed the US frigate Samuel B. Roberts in the central Gulf on April 14, 1988, was estimated to cost about $1,500. Repairing the ship cost about $90 million.

The spikes on an old-style mine are its trigger: when a ship’s hull strikes one and bends it, it closes the firing circuit and the charge goes off.

Those horns belong to the contact mine, the oldest working design, which needs a ship to physically hit it. The Soviet-pattern M-08 contact mines that Iran laid in the 1980s were of this type. A contact mine has to be moored near the surface, where a hull will meet it, and that is also its weakness: it can be found, and a sweep can cut its mooring cable.

An influence mine has sensors instead of horns, and it fires when a passing ship’s magnetic field, sound or pressure matches what it is set to detect.

There are three main kinds of sensor. A magnetic sensor picks up the way a steel hull disturbs the earth’s magnetic field. An acoustic sensor listens for the noise of engines and propellers. A pressure sensor registers the drop in water pressure under a large ship moving overhead. Many modern mines combine two of them, most often magnetic and acoustic, and some carry a ship counter that lets the first few ships pass before the mine fires. That is why a lane that ships have already crossed safely is not proof that it is clear.

Navies answer by shrinking their own signatures. Steel warships carry degaussing coils that cancel much of their magnetic field, and the US Navy’s Avenger-class minehunters were built with wooden hulls sheathed in fiberglass so they could work inside a minefield. Iran’s own influence types include the moored Maham-3, which listens for a ship’s sound, and the Maham-2 and Maham-7 bottom mines.

Almost anything that floats or flies can lay mines: warships, converted cargo vessels, small boats, submarines and aircraft.

That is what makes them so hard to stop. In September 1987, US forces caught the Iranian landing ship Iran Ajr laying M-08 contact mines at night near Bahrain. In this war Iran has relied on small boats. US Central Command said it destroyed 16 Iranian minelayers near the Strait of Hormuz on March 10, 2026. Iran has also shown helicopters dropping mines in exercises.

A limpet mine is laid differently, by someone getting close enough to a ship to attach it. The tanker set on fire off Khasab, Oman, on October 29 was hit by two of them, as we reported in the Khasab limpet mine attack.

Navies clear mines in two ways: sweeping, which sets them off or cuts them loose from a distance, and hunting, which finds each mine and destroys it where it lies.

A mechanical sweep is a wire towed behind a ship, fitted with cutters that sever the cables of moored mines. The freed mines float to the surface, where they are destroyed by gunfire or by bomb-disposal teams. An influence sweep imitates a ship: towed cables carry an electric current to create a magnetic field, and towed noisemakers copy a ship’s sound, so the mine fires harmlessly behind the sweeper. Pressure mines are the hardest to sweep, because a ship’s pressure signature is difficult to fake.

Hunting is slower but more certain. Sonar on a ship, helicopter or underwater drone finds mine-like objects, a diver or remotely operated vehicle identifies each one, and a charge is placed on or beside it. The US Navy is moving that work onto unmanned systems carried by littoral combat ships, with only four dedicated Avenger-class minesweepers left, as our explainer on how many minesweepers the US Navy has shows.

Clearing a field takes time. Off Wonsan, Korea, in October 1950, North Korea laid about 3,000 mines, and two US minesweepers, the Pirate and the Pledge, were sunk on October 12 while clearing a path for a landing that the minefield held up for days.

Yes. Naval mines are not banned, but the Hague Convention of 1907 on submarine contact mines limits how they may be used.

The convention forbids laying unanchored contact mines unless they become harmless within one hour after the side that laid them loses control of them. It forbids anchored mines that stay dangerous after breaking loose from their moorings, and laying contact mines off an enemy’s coast and ports with the sole aim of stopping commercial shipping. A party that lays anchored mines must take every possible precaution for the safety of peaceful shipping, warn shipowners of danger zones when the mines are no longer watched, and remove its mines when the war ends. The one-hour rule is aimed at drifting mines, which can travel far from where they were laid and threaten any ship in their path. The treaty was written before influence mines existed, and its rules address contact mines only.