Deep-sea explorers said Friday they have mined what could be the richest shipwreck treasure in history, bringing home 17 tons of colonial-era silver and gold coins from an undisclosed site in the Atlantic Ocean. Estimated value: $500 million.

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 are 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...