Deep-sea explorers have encountered what seems to be the richest sunken treasure ever found: about 500,000 of colonial-era silver and gold coins, which could worth $500 million.

The trove was discovered on a shipwreck in the Atlantic Ocean. "For this colonial era, I think [the find] is unprecedented. I don't know of anything equal or comparable to it", said rare coin expert Nick Bruyer, who was contracted by Tampa-based Odyssey Marine Exploration to analyze samples of coins from the wreck.

For security reasons, the company did not offer any details about the shipwreck site.

It seems that the coins were found on a wreck of a 17th-century merchant ship off southwestern England. "Because the shipwreck was found in an area where many colonial-era vessels went down, the company is still uncertain about its nationality, size and age, although evidence points to a specific known shipwreck. The site is beyond the territorial waters or legal jurisdiction of any country. Rather than a shout of glee, it's more being able to exhale for the first time in a long time," said OME co-founder Greg Stemm.

The discovery is by far the biggest in the company's 13-year history.

The company's officials did not mention if the treasure was found in the same 17th-century wreck site near the English Channel, situated at about 40 miles (64 km) off the Southwestern England, for which they petitioned a federal court for permission to salvage, received on Wednesday.

This was part of the secretive project "Black Swan," and the company did not mention details of the new coins, such as their type, denomination or country of origin. "I observed a wide variety of coins that probably were never circulated. The currency was in much better condition than artifacts yielded by most shipwrecks of a similar age. The coins - mostly silver pieces - could fetch several hundred to several thousand dollars each, with some possibly commanding much more," said Bruyer.

"It's absolutely impossible to accurately determine the value without knowing the contents and the condition of the retrieved coins. It's like trying to appraise a house or a car over the phone," said Donn Pearlman, a rare coin expert and spokesman for the Professional Numismatists Guild.

A coin's high value comes from its rarity, condition, story behind, controlled release into the market and aggressive marketing.