Mine clearance is being discussed this week as though it were a task with a completion date. It is one of the slowest and most conditional operations any navy undertakes. Here is what it actually involves.

What a sea mine is

A naval mine is an explosive charge placed in the water and left to wait. It is the cheapest weapon in naval warfare relative to what it can destroy, and its main effect is usually psychological rather than physical: a strait believed to be mined stops being used, whether or not any ship is actually hit.

Three broad types matter here.

• Moored contact mines float below the surface on a cable anchored to the seabed, and detonate when a hull strikes them. The design is more than a century old and remains effective.
• Bottom influence mines sit on the seabed and detonate when they sense a ship’s magnetic field, engine noise or pressure signature. They can be set to ignore the first several ships and fire at the seventh, which makes proving an area clear far harder.
• Drifting mines are not anchored. They move with current and wind, which makes them indiscriminate and their position unknowable.

Sweeping versus hunting

There are two approaches, and modern navies use both.

Minesweeping is the older method. A vessel tows equipment that cuts the mooring cables of anchored mines so they float up to be destroyed by gunfire, or that generates a false magnetic or acoustic signature to trigger influence mines at a safe distance. It is relatively quick and covers area, but it cannot confirm what it missed.

Minehunting is the slower method and the one that produces confidence. Sonar searches the seabed and the water column for objects, each contact is classified, and the ones that cannot be ruled out are inspected individually — by remotely operated vehicle, by autonomous underwater vehicle, or by clearance divers. Anything identified as a mine is destroyed in place with a small charge.

The limiting factor is that seabeds are full of objects. Wrecks, containers, oil drums, rocks and discarded machinery all return sonar contacts. In a busy commercial waterway, an operation can spend most of its time confirming that debris is debris.

Why “clear” is a probability, not a state

This is the part that matters for reading the news.

Navies do not declare water free of mines. They declare a clearance level — a stated probability that a mine remains in a defined route, given the equipment used and the area searched. A route might be cleared to a high confidence for vessels of a certain draught and still not be declared safe for others.

Several factors make even that difficult. Bottom mines sink into silt and vanish from sonar. Ship-count fuses mean a mine can lie inert through a clearance operation and detonate later. And a hostile party can re-lay mines behind a clearance operation from a fishing boat.

How long it takes

Historical clearance operations offer a rough sense of scale. After the Second World War, clearing European and Asian waters took years, and some fields were never fully cleared — fishing vessels in the North and Baltic Seas still recover ordnance from that period. After the 1991 Gulf War, coalition forces spent months clearing mines laid off Kuwait.

Two things speed it up: knowing where the mines were laid, and having the party that laid them cooperate. Minefield records turn a search problem into a retrieval problem. That is the practical significance of a joint clearance project — not the ships, but the charts.

The insurance test

In commercial terms, a waterway reopens when underwriters price war-risk cover at a level shipowners will accept — not when a government announces it is safe.

Underwriters look at clearance levels, at who verified them, at whether an independent body has surveyed the route, and at whether the party that laid the mines has an interest in the route staying open. This is why the phrase to watch in coverage of the Strait of Hormuz is not “cleared” but “war-risk premium” — the number that reveals what the people taking the risk actually believe.

Related coverage

• Iran and Oman Propose a Corridor Through Hormuz. Tehran Says the Strait Is Still Closed
• Fact Check: Have All the Mines Really Been Cleared From the Strait of Hormuz?