It is 10 years since a deal to protect the world's thousands of shipwrecks, but the UK and several other major maritime powers are yet to ratify it. Should this underwater heritage be protected or is it acceptable to plunder?

When a ship sinks and lives are lost, it is a tragedy for the families involved.

For the relatives of the dead, the ship becomes an underwater grave but as the years pass the wreck can become a site of archaeological interest.

In recent years technological innovations have allowed commercial archaeologists, decried by some as "treasure hunters", to reach wrecks far below the surface.

The most famous of them all, the Titanic, is nearly three miles down and to get thereas film director James Cameron has shown,involves using "robot" divers which are prohibitively expensive - around $50,000 (£32,000) a day.

Unesco convention Adopted in Nov 2001 but only came into force in 2009 when the 20th nation - Barbados - ratified it

Designed to complement the 1982 UN Law of the Seas, which failed to mention shipwrecks

40 nations have ratified it but the only major seafaring nations are Spain and Portugal

The convention only covers wrecks or ruins which are over 100 years old

States are responsible for ships in their territorial waters but for wrecks in international waters a number of "interested parties" could be involved

Salvage firms are most interested in ships with cargoes of gold and silver, ceramics or other valuables.

In November 2001, theUnesco Convention on the Protection of Underwater Cultural Heritagewas finally adopted.

But 10 years on, it still has not been ratified by the UK, France, Russia, China or the US, and commercial archaeologists continue to locate wrecks, remove their cargoes and sell them off.

"The convention has not been ratified yet because of the issues it throws up about the cost of implementing and policing it," a spokesman for the UK Department for Culture, Media and Sport, says. "Discussions continue within government, but ratification is not currently seen as a priority."

In September Britain's Department of Transport announced it had signed a deal withOdyssey Marine Explorationfor the salvage of 200 tonnes of silver, worth up to £150m, from the SS Gairsoppa, which was sunk by a German U-Boat in 1941.

The British government will get 20% of whatever Odyssey recovers but Unesco says the deal broke the spirit of the convention.

Image caption The wreck of Captain Kidd's pirate ship was discovered off the Dominican Republic

Robert Yorke, chairman of the Joint Nautical Archaeology Policy Committee, argues the real reason the government, and the Ministry of Defence in particular, are not ratifying the convention was because of a misplaced fear about the implications for British warships around the world.

The internationally recognised concept of "sovereign immunity" means nations should not interfere with foreign warships.

Under the Military Remains Act 1986,a number of British warships around the world are protected, including several ships sunk during the Falklands conflict. Also covered are several German U-boats in UK waters.

There are an estimated three million wrecks on the seabed.

Unesco believes attitudes to the exploration of wrecks are out of step with land archaeology.

"The looting of the tombs of Tutankhamen is now considered unacceptable, so why is the looting of shipwrecks considered acceptable?" says Unesco's Tim Curtis.

Caesar Bita is a Kenyan maritime archaeologist and an expert on ancient trade between China and Africa.

Image caption This gold pendant, from South America, was found on the Nuestra Senora de Atocha off Florida

He believes he is close to finding the remains of the legendary fleet of Zheng He. According to stories, a ship from the Chinese admiral's fleet is thought to have foundered off the coast of Kenya in the early 1400s.

Bita says the wrecks could provide evidence of early contact between China and East Africa.

"Shipwrecks are always under threat all over the world by people collecting material from the site and the situation in Kenya is not unique," he says.

Sean Fisher, whose grandfather Mel discovered the treasure ship Nuestra Senora de Atocha off Florida, says he is not "ashamed to call himself a treasure hunter."

"Purist archaeologists turn up their noses at us," he says. "But every artefact we find, whether it's a piece of pottery, a gold bar or a spike used in the rigging gets treated with exactly the same care.

"Everybody loves gold and everybody has a bit of treasure hunter in them but for me the most exciting thing I ever found was a 400-year-old arquebus (hook gun). It was like bringing history back to life."

But the idea that mass heritage is at risk is scaremongering, says Dr Sean Kingsley, a director of Wreck Watch International and a spokesman for Odyssey.

"Sand dredging, offshore fishing, pipeline laying, scuba-diving trophy hunters and governments' failures to police these industries are the true greatest threats to the world's shipwrecks."

He argues that the nations which have ratified the convention represent only 5% of the world's coastline.

Image caption Sonar surveys picked up the outline of the Wilhelm Gustloff in the Baltic, the grave of 9,500 people

The convention only covers wrecks that are over 100 years old, which means the Titanic will only be covered from next year and ships from World War I and II have no protection.

That is something which concerns naval veterans.

Last month seven European naval associationswrote a letter to The Timesto protest at Dutch salvage firms who they said were "desecrating" the graves of three British warships, which were torpedoed off the Netherlands in 1914, in their search for scrap metal.

But some wartime wrecks have been protected.

The Polish Maritime Office recently placed a 500m diving exclusion zone around the wreck of the Wilhelm Gustloff, which sank in the Baltic in January 1945. The ship, packed with 10,000 German refugees from the Eastern Front, was sunk by a Soviet sub. Only 500 survived and it is the single largest death toll at sea.

In 2006 Australian divers located the wreck of aJapanese mini-submarine, M24,three miles off Sydney.The sub, which is believed to contain the bodies of two young Japanese submariners, came to grief after taking part in an attack on Sydney harbour in 1942.

The Australian authorities placed a similar 500m zone around the wreck, monitored by sensitive hydrophones, with a A$1.1m (£725,587) fine for anyone who interferes with the wreck.

Maritime archaeologist Mark Wilde-Ramsing is unlikely to get to meet the relatives of those who died on the wreck he is exploring.

Image caption This Spanish seal was found on the San Diego, which sank in the Philippines in 1600

The Queen Anne's Revenge was an English pirate ship commanded by Edward Teach, better known as Blackbeard, which sank off the coast of North Carolina in 1718.

Many of theexhibits- including pistols, rum bottles, slave shackles and a surgeon's kit - are now on display at a museum in North Carolina but sadly Blackbeard's treasure was nowhere to be found.

But the convention does not just apply to shipwrecks.

It also covers archaeological sites which are now underwater, such asPort Royal in Jamaica,a former pirate harbour once described as the "wickedest city on Earth", as well as prehistoric sites in the North Sea, and the watersoff Alexandria, in Egypt.

But if wrecks and ruins continue to fascinate, what is the best way to satisfy the public curiosity?

In 1982 the Mary Rose, flagship of England's King Henry VIII, was raised from the Solent and next yeara £35m museum will open in Portsmouth.

Some argue it would have been better, in hindsight, for the Mary Rose to have stayed on the seabed, because of the expensive chemical treatment needed to preserve the timbers.

Image caption Shipwrecks can be significant sites for archaeologists

"It has created the myth that all shipwrecks are bottomless money pits and that hull and mass artefact recovery are best avoided," suggests Dr Kingsley.

But Prof Jon Adams, head of maritime archaeology at the University of Southampton, strongly disagrees.

"If it was falling to pieces and nobody came to see it I'd agree, but its conservation has been a highly successful research project in its own right and it is one of the most popular maritime museum in world with over 300,000 visitors a year," he says.

Many experts believe the future is in underwater trails or virtual museums, where the wreck remains in situ and cameras relay real-time pictures to a museum on the surface.

Australia is also home to a number of "underwater heritage trails", with plaques offering information for divers.

In the Dominican Republic a "living museum" has been set up around the wrecks of two Spanish galleons which sank during a hurricane in 1724.

The living museum was the idea of Prof Charles Beeker, of Indiana University.

"We want people to come and visit but to take only pictures and leave only bubbles," he says.

Prof Beeker, who has also discovered the wreck of Captain Kidd's pirate ship the Quedagh Merchant, said all divers had to be "sensitive" to the fact that wrecks are essentially graveyards and he criticised some who took the skulls of Japanese seamen from the many wrecks in Truk Lagoon in the Pacific.

Image caption The Mary Rose, which was excavated from the Solent in 1982, will go on display in a new museum

While it pushes in situ preservation, Unesco is hopeful several major countries, including Australia and France, might soon ratify the convention to give it more weight.

Dr Kingsley is doubtful and says self-regulation is the best way forward: "The future is going to be an expensive and unimaginable journey, a challenge best met by sharing ideas, information and enlightened management, not by using the Unesco convention to slap parking tickets' on robots' windows."

But Prof Adams says self-regulation does not work and added: "The Unesco convention represents best practice and is the only feasible way of protecting underwater cultural heritage in international waters."