HIDDEN in the cold depths of the Baltic close to the port of Tallinn are tons of deadly high explosives.

Massive ferries run though the area. Fishermen trawl there, and occasionally catch more than they bargained for, bringing up one of the mines or other explosive items remaining in Estonian waters as a legacy of the decades when Estonia was enveloped in the twilight of war and Soviet occupation.

While the risk of one of these weapons exploding is normally slight, the destructive powers of mines should not be underestimated - when the hi-tech American warship USS Princeton set off a mine beneath her in the Gulf War in 1991, it did hundreds of millions of dollars worth of damage to the hull and electronic systems.

It is estimated that more than 80,000 mines were laid in the Eastern Baltic during the two World Wars alone, with other munitions - practice bombs, torpedoes, depth charges and the like - being added between 1944 and 1994, the years when Soviet armed forces used the coastline of Estonia, Latvia and Lithuania as a live-firing range.

Many of these mines were soon cleared - the task of sweeping vast fields of moored buoyant mines was relatively straightforward.

But some ground mines, torpedoes, sunken moored mines and other ordnance remains, and no one knows in what state - a mine from World War I, made of good-quality steel, would rust slowly in these cold seas, presenting a very real danger to fishermen and other merchant mariners.

Against this background came an invitation, in 1998, for the Royal Navy, acknowledged world-leaders in mine countermeasures (MCM) and unexploded ordnance operations, to help clear the area.

As a consequence, HMS Atherstone, as part of a Swedish/Estonian operation, identified and destroyed 12 mines out of a total of 52 found by the force.

In October 2000, HM ships Quorn and Chiddingfold joined 16 other NATO/Baltic ships in clearing 72 items of ordnance - a total of 75 tons of high explosives - from the Gulf of Riga, and last year HM ships Bangor, Grimsby and Ledbury all joined in, Bangor and Grimsby clearing 21 items between them in May.

The shallow Baltic is not an easy sea to work in. Storms can force the ships to abandon delicate mine clearance operations, and the cold water - temperatures are around 2 degrees C in the late autumn - has at times a visibility of only three or four feet.

The mine clearance operations staged during November in the southern part of the Gulf of Finland were the largest of their type in more than 20 years.

Of the 1,000 sailors in 24 ships from ten nations, 100 men and women were provided by the Royal Navy, led by Cdr Charlie Wilson.

Cdr Wilson was in charge of a five-strong multi-national group which included HMS Penzance, one of the newest of the Navy's Sandown-class single-role minehunters.

Sister ship HMS Pembroke worked with a Baltic minesweeping squadron under the command of an Estonian officer, while HMS Bridport took part as the UK contribution to NATO's MCM Force North.

Part of the Outreach programme, supporting the fledgling democracies of central and eastern Europe, these ongoing operations in the Baltic also give the Royal Navy a valuable opportunity to maintain its pre-eminence in mine warfare techniques.

Over the course of the two-week programme in November, the combined forces of the UK, Estonia, Belgium, Denmark, Germany, Latvia, Lithuania, the Netherlands, Norway and Sweden found 74 items of ordnance - 47 mines, 21 'miscellaneous projectiles', three torpedoes, two smoke bombs and one depth charge.

Although the removal of maverick explosives is central to the operation, the presence of the British minehunters, and the way they conduct their business, is just as important in terms of international relations.

The importance of the operations was underlined by the visit of Prince Michael of Kent, Honorary Commodore of the Royal Naval Reserve, to see the ships at work and speak to the regulars and reservists manning them.

The British are particularly welcome in these parts, a throwback to the immediate aftermath of World War I.

Estonia had seceded from Russia after the October Revolution of 1917, but was immediately invaded by the Bolsheviks from St Petersburg, then in turn by the Germans moving up from the south - yet in the interval between the two occupations, she had declared herself fully independent.

In the uneasy endgame of the war, Estonia canvassed support from the Allied powers, playing on fears that the Germans might use the Gulf of Finland as a means to capture thousands of tons of war stores which had been stockpiled at Murmansk.

Estonia was recognised in May, 1918, but it was not until after the Armistice in November that Britain could support the state in any meaningful way - with Naval units based in the familiar surroundings of Reval (Tallinn), a highly-effective Royal Navy submarine base during the war, and which now fulfilled a UK need for a Baltic base.

Sporadic gun actions between the British and Bolshevik Baltic Fleet, which supported attempts at Russian invasion along the coast, broke out in early 1919.

But after spectacular reverses - a cruiser and two battleships were sunk by British coastal motor boats - Lenin dropped the matter of Estonia as a consequence of British naval support, and in February 1920, Soviet Russia and Estonia signed the Treaty of Tartu, relinquishing all Russian claims.

Although not widely appreciated by people in the UK, the helping hand given by the Royal Navy to the Baltic state is still remembered with gratitude in Estonia, and the current operations, although not in the same league, demonstrate a continuing commitment to the country.

There is also a spin-off for personnel on board the British ships - although it is usually cold and gloomy at this time of the year in northern latitudes, the historic links between the UK and Estonia make Tallinn a good run ashore and usually guarantees British matelots a warm welcome.

Another major consideration during mine clearance operations was the safety of marine wildlife in the area.

Advice was sought from local civic authorities and environmental groups before specific operations, and steps were taken where it was deemed necessary to move rare or sensitive species well out of range before mine disposal charges were set off.

In addition, before any explosives are fired, minehunters transmit on their high-frequency sonar, which will tend to temporarily drive aquatic life away from the immediate vicinity, and a good visual lookout has to be maintained from the deck by members of the ship's company.

Mine countermeasures operations tend to fall into two distinct categories.

Minesweeping techniques have hardly changed since World War I - a ship tows equipment astern to cut the wires which moor buoyant mines in position, or set them off by imitating the magnetic or sound signature of a target ship.

But that process puts the minesweeper in danger, and modern technology means it is not foolproof - a minefield may contain different mines, some on the sea-bed, and built-in computers may only activate a mine when a specific ship passes over, or a number of contacts have been counted through.

The preferred option is to hunt for individual mines using sonar, treating the objects as tiny, stationary submarines.

When a mine is located, a remote controlled submersible - the Yellow Submarine - is launched to take a closer look, and, if necessary, lay explosive charges to detonate the mine.

Divers may also be called on in particular situations, for example to help with identification, but that would only be considered when the task cannot be undertaken by a remote controlled vehicle, as the safety of personnel is the overriding priority.