Florida-based treasure hunters Odyssey Marine Exploration said on Friday it had legally recovered gold and silver coins worth an estimated 500 million dollar from a colonial-era wreck code-named Black Swan at an undisclosed location in the Atlantic Ocean.
Spain's Culture Ministry called the discovery suspicious and said the booty could have come from a wrecked Spanish galleon or the remains of HMS Sussex off the coast of Gibraltar, which Spain and Britain have allowed Odyssey to survey.
''Both taking (treasure) from Spanish waters or a Spanish vessel that sunk outside Spanish waters would be illegal,'' a
spokeswoman for the ministry said.
''The Culture Ministry reported the suspicion of possible criminal plundering and we alerted the civil guard (police) who are investigating.'' Gibraltar's government said the haul had nothing to do with the Sussex, a British flagship which sank in a storm in 1694.
But Ministry of Defense personnel in Gibraltar said an Odyssey ship berthed in Gibraltar's naval dock last week and a specially chartered American Airlines plane landed in the British colony on Wednesday before loading cargo and flying to Tampa, Florida, where Odyssey is headquartered.
Odyssey said it knew nothing of a Spanish investigation and said that at no point did its recovery mission come within Spanish jurisdiction a point it would be happy to prove.
''The recovery was conducted in conformity with Salvage Law and the Law of the Sea Convention, beyond the territorial waters or legal jurisdiction of any country,'' the company said in a statement.
''We do not believe that the recovery is subject to sovereign immunity by any nation pursuant to the Law of the Sea Convention.'' The coins were brought to the United States with a ''valid export license granted by the country from which they were exported, and imported legally pursuant to US law,'' it said.
Odyssey said it would announce the identity of the shipwreck only after it had been confirmed by research and would release more details of the coins once they had all been documented.
Research suggests the 80-cannon Sussex was on a secret mission to northern Italy to deliver a massive cargo of coins to the Duke of Savoy in return for fighting France.
The duke eventually changed sides, which historians speculate was because he failed to receive the British payment and instead gave in to French bribes.
British newspapers have speculated that Odyssey's mystery treasure ship was the Merchant Royal which sank just off the southern tip of England in 1641.
However, the area has always been packed with shipping routes and could have been used by Spanish galleons taking gold and silver to the Spanish Netherlands or even by the Spanish Armada as it sailed to attack England in 1588.
News of the discovery, which yielded gold and over 17 tonnes of silver coins, shot Odyssey's shares more than 60 per cent higher on Friday. They were up another 6 per cent on Monday.