It may be the richest shipwreck treasure in history: some 15 tonnes of colonial-era silver and gold coins from an undisclosed site in the Atlantic Ocean.

On Friday, the exploration and salvage company that found the wreck estimated the value at $500 million US.

The name and location of the wreck salvaged by Odyssey Marine Exploration are carefully guarded secrets.
(Jonathan Blair/Associated Press)

A jet chartered by Tampa-based Odyssey Marine Exploration landed in the United States recently with hundreds of plastic containers brimming with coins raised from the ocean floor, Odyssey co-chairman Greg Stemm said.

The more than 500,000 pieces were expected to fetch an average of $1,000 each from collectors and investors.

"For this colonial era, I think (the find) is unprecedented," said rare coin expert Nick Bruyer, who examined a batch of coins from the wreck. "I don't know of anything equal or comparable to it."

Citing security concerns, the company declined to release any details about the ship or the wreck site Friday. Stemm said a formal announcement will come later, but court records indicate the coins might come from a 400-year-old ship found off England.

Because the shipwreck was found in a lane where many colonial-era vessels went down, there is still some uncertainty about its nationality, size and age, Stemm said, although evidence points to a specific known shipwreck. The site is beyond the territorial waters or legal jurisdiction of any country, he said.

"Rather than a shout of glee, it's more being able to exhale for the first time in a long time," Stemm said of the haul, by far the biggest in Odyssey's 13-year history.

He wouldn't say if the loot was taken from the same wreck site near the English Channel that Odyssey recently petitioned a U.S. federal court for permission to salvage.

In seeking exclusive rights to that site, an Odyssey lawyer told a judge last fall that the company likely had found the remains of a 17th-century merchant vessel that sank with valuable cargo aboard, about 65 kilometres off the southwestern tip of England. A judge signed an order granting those rights last month.

In keeping with the secretive nature of the project dubbed "Black Swan," Odyssey also isn't talking yet about the types, denominations and country of origin of the coins.

Bruyer said he observed a wide range of varieties and dates of likely uncirculated currency in much better condition than artifacts yielded by most shipwrecks of a similar age.

The Black Swan coins — mostly silver pieces — likely will fetch several hundred dollars to several thousand dollars each, with some possibly commanding much more, he said. Value is determined by rarity, condition and the story behind them.

Controlled release of the coins into the market along with their expected high value to collectors likely will keep prices at a premium, he said.

The richest ever shipwreck haul was yielded by the Spanish galleon Nuestra Senora de Atocha, which sank in a hurricane off the Florida Keys in 1622. Treasure-hunting pioneer Mel Fisher found it in 1985, retrieving a reported $400 million in coins and other loot.

Odyssey likely will return to the same spot for more coins and artifacts.

"We have treated this site with kid gloves and the archeological work done by our team out there is unsurpassed," Odyssey CEO John Morris said. "We are thoroughly documenting and recording the site, which we believe will have immense historical significance."

The news is timely for Odyssey, the only publicly traded company of its kind.

The company salvaged more than 50,000 coins and other artifacts from the wreck of the SS Republic off Savannah, Ga., in 2003, making millions. But Odyssey posted losses in 2005 and 2006 while using its expensive, state-of-the-art ships and deep-water robotic equipment to hunt for the next mother lode.

"The outside world now understands that what we do is a real business and is repeatable and not just a lucky one-shot deal," Stemm said. "I don't know of anybody else who has hit more than one economically significant shipwreck."

In January, Odyssey won permission from the Spanish government to resume a suspended search for the wreck of the HMS Sussex, which was leading a British fleet into the Mediterranean Sea for a war against France in 1694 when it sank in a storm off Gibraltar.

Historians believe the 48-metre warship was carrying eight tonnes of gold coins to buy the loyalty of the Duke of Savoy, a potential ally in southeastern France. Odyssey believes those coins could also fetch more than $500 million.

But under the terms of a historic agreement, Odyssey will have to share any finds with the British government. The company will get 80 per cent of the first $45 million and about 50 per cent of the proceeds thereafter.