By Kathryn Westcott
The interception of a treasure-hunt ship off the coast of Gibraltar is the latest broadside in a tense battle between a US-based salvage company and Spain over an unidentified shipwreck and its rich cargo of gold and silver coins.
On Tuesday, patrol boats from Spain's maritime police intercepted the 76m Odyssey Explorer, owned by underwater salvage firm Odyssey Marine International, three miles off the coast of Gibraltar. It was ordered to the Spanish port of
Algeciras for inspection.
Odyssey Explorer is followed by a patrol boat of the Guardia Civil
Spain's Guardia Civil has been keeping a close eye on the company's vessel since a Spanish judge ordered that it be detained and searched if it left port in Gibraltar.
The company says its recovery vessel has been effectively blockaded since the ruling in June. Spain believes it could provide clues to the identity and location of the wreck that yielded half-a-million colonial era silver and gold coins.
It suspects that a Spanish galleon is being secretly plundered - or that the wreck lies in Spanish waters.
Odyssey Marine Explorations - which became the most famous deep water treasure hunting company when it announced the discovery last May - says it is keeping the location of the wreck secret, to protect the site from looters.
All it is saying is that the wreck - codenamed Black Swan - is somewhere in the Atlantic.
The haul, which has an estimated value of $500m (£350m), is now at a secret location in Florida, where Odyssey is based.
A court in the state is currently considering motions filed by the company and by Spain concerning the ownership of the booty.
Greg Stemm with the Black Swan haul at a secret location in the US
Treasure hunters have long dreamed of discovering hauls of gold and silver in the western Mediterranean. The area is a graveyard of French, Spanish and British galleons and warships sunk by storms and pirates during Spain's long dominance of the sea.
Once the domain of schoolboy fantasies, the hunt for treasure on the deep ocean floor has become big business for companies like Odyssey. And the company's recent listing on the Nasdaq indicates that it is a business that investors are prepared to take seriously.
Odyssey has several shipwreck projects in various stages of development around the world, but its involvement with the Spanish goes back almost a decade.
"Odyssey has had an excellent working relationship with the Spanish Government for many years," Greg Stemm, co-founder of Odyssey Marine Exploration told the BBC News website.
RULES OF THE WAVES
Seas and oceans governed by UN Laws of the Sea
Ownership of sunken property in international waters governed by the law of salvage and the law of finds
Law of salvage: If property is owned, those finding it are entitled to compensation for their salvage efforts
Law of finds: Salvor is entitled to all reclaimed property if it is proved to be abandoned
Spain insists it retains rights to all its sunken treasure
Odyssey intends its haul to be dealt with under US federal law, where previous judgements have sometimes granted exclusive rights to salvors
"We have always respected Spain's interest in its maritime heritage and have therefore consistently communicated our activities to Spain."
The relationship has soured over the past few months. In July, another of Odyssey's vessels was stopped and forcibly boarded as it tried to leave Gibraltar. A computer was confiscated.
The company told the BBC News website that it intends to seek compensation from Spain for revenues lost because of the kingdom's intervention in its activities.
The battle over the Black Swan treasure is now jeopardising another more lucrative project: the salvage of a British warship that Odyssey believes it has discovered in the western Mediterranean.
The British Government is collaborating with Odyssey to recover the warship, thought to be HMS Sussex, which went down in a storm off Gibraltar in the Mediterranean Sea in 1694.
According to the Council for British Archaeology, it was on its way to provide British financial support to the Duke of Savoy during the war against Louis XIV. The council says she was believed to have been carrying bullion, which is estimated by some experts to be worth some £2.5bn ($5bn) today.
The warship apparently lies in waters that Britain and Gibraltar claim are international but that Spain claims as its own.
Diplomatic talks resulted in an agreement being reached with Spain earlier this year but the project is on hold.
A spokeswoman for the Foreign and Commonwealth Office told the BBC News website that the "on-going court case between Spain and Odyssey should be resolved first."
It is anyone's guess how long this will take.
This will no doubt please archaeologists - including the Council for British Archaeology - who were enraged by the deal.
Experts describe such activities as commercial treasure hunting under the guise of archaeology, arguing that a dangerous precedent would be set allowing private firms to profit from historic wrecks.
And in Spain, Odyssey's activities have been painted as modern-day piracy.
The company acknowledges that its primary concerns are commercial but it also has a mission statement that sets out how it also wants to do good archaeology.
It maintains that it works to the highest of standards, employing experts and archaeologists.
"Our contract with the United Kingdom sets an excellent example of how such a collaboration between the public and private sector can produce excellent archaeological work," says Mr Stemm.
The eight-tonne robot Zeus can reach depths of up to 2,500 metres
And he says that the model - the first of its kind with a government - could be extended to other countries, including Spain.
Odyssey undertakes multi-million-dollar operations, deploying sophisticated deep-sea technology and robotics to scour the ocean beds.
On board the Marine Explorer, for example, is a $4m underwater robot Zeus, which deploys an array of brilliant strobe lights and cameras as it carefully picks through debris at depths of up to 2,500m.
The company's first major salvage venture was in 2003 when it discovered the SS Republic, a Civil War side-wheel steamer that sank off the Florida coast in 1865 and some $75m (£37m) worth of coins.
Odyssey has now filed finders-keepers' claims with a court in Tampa, Florida US on the Black Swan and two other shipwrecks.
But Spain is challenging these arguing that the company is withholding crucial information. It is also claiming a right to share the treasure.
Jim Goold, a maritime lawyer in Washington representing the Spanish government, told the BBC News website that the implications of the case are huge in a era when cutting-edge technology is bringing new gravesites to light.
"Here you have a situation in which a US company is apparently systematically working in the Mediterranean, the English Channel and off the coast of Spain, taking cultural heritage without authorisation and then whisking it to the US and steadfastly refusing to reveal to the concerned governments what it has been doing," he said.
Odyssey says the Black Swan recovery was conducted in conformity with Salvage Law and the Law of the Sea Convention, beyond the territorial waters of legal jurisdiction of any country.
It expects to reap a substantial salvage award regardless of who claims the treasure.