The submarine as a weapon is over 200 years old, but its history and development have been fraught with accidents and mishaps that have often ended with tragic loss of life.

One of the earliest submarines, the Hunley, was developed by the Confederates during the US Civil War.

Named after her inventor, Horace Lawson Hunley, and little more than a long metal tube, the 'boat' sank twice during trials, before being raised to mount an attack on a US Federal ship, the USS Housatonic.

Although the mission was successful, she was caught in the blast from the primitive spar-mounted torpedo and sank once more with all hands.

Hazards

During the half-century, as technology improved, many countries began to explore the submarine's possibilities as a weapon.

Many sailors have never resurfaced

But the process remained one of trial and error, and early submariners still faced a huge number of potential hazards.

Gas and acid leaking from primitive batteries, exploding torpedoes and collisions cost the lives of several thousand sailors world-wide.

However, despite these problems, the effectiveness of the submarine was proved during the First World War, with German U-boats inflicting huge losses on Allied shipping.

In the inter-war years, most nations concentrated on building bigger, better and more deadly submarines.

Progress

But this process was still fraught with dangers, and many countries' drives to build up their fleets and experiment with different designs resulted in further accidents.

In 1939, a faulty torpedo tube sank the British sub Thetis in Liverpool bay killing 100 men.

Other designs provided submarines with enormous deck-mounted guns, such as the British 'X-1', or even aircraft - often with disastrous consequences.

Submarines again saw extensive action and development during World War II, primarily in an anti-shipping role.

Submarines have become key parts of nuclear arsenals

But the next stage of the submarine's evolution came with the advent of the atomic age.

Nuclear power gave submarines virtually unlimited range, and allowed them to run almost silently and undetected through the world's oceans.

Submarines' roles also changed from being primarily anti-shipping, to acting as ocean-going platforms for huge arsenals of ballistic missiles.

But nuclear power proliferation beneath the waves has inevitably led to disturbing repercussions.

Losses of a nuclear powered vessels are always more serious, as they raise the threat of irreparable damage to marine life from radioactive contamination.

Mysterious circumstances

Six nuclear submarines - including the Kursk - now lie on the ocean floors.

Nuclear sub losses

1963: USS Thresher sinks off the coast of New England 1968: USS Scorpion lost in the mid-Atlantic 1970: Soviet November class sub sinks 480km north-west of Spain 1986: Soviet sub sinks 960km north-east of Bermuda 1989 : Soviet sub Komsomolets sinks off northern Norway 2000 : Russian Oscar class sub Kursk sinks in the Barents sea

Some have been lost in mysterious circumstances and in deep-water locations, with their reactors and missiles remaining unaccounted for.

The first nuclear sub to be lost at sea was the USS Thresher, which was lost off Cape Cod in April 1963 with the loss of 129 lives.

A faulty welding leading to disastrous flooding was blamed for the boat's loss.

In 1968, another US boat, the USS Scorpion was lost in the mid-Atlantic en-route to the Canary Islands.

Although the Scorpion's sinking was blamed on a faulty torpedo circling and hitting the vessel, there has long been speculation that the boat could have been involved in an incident with an unknown Russian sub.

Forty two sailors died when the Komsomolets sank in 1989

Since the 1960s a further three Russian submarines have also been lost, many of the details of which were kept well secret for over 30 years.

Some scientists say the lost boats represent little or no risk to the environment, pointing to most of the lost vessels lying in depths of more than 15,000ft.

But others worry that as the number of nuclear reactors and missiles littering the world's oceans increases, so too does the threat of serious environmental damage.