The SS Republic, a paddlesteamer that had survived service in the Civil War, was sailing from New York to New Orleans in October 1865 with a cargo of 20,000 $20 gold coins for the recently defeated financial capital of the South when it was overtaken by a “perfect hurricane”.

After two days battling against the storm 100 miles southeast of Savannah, Georgia, the engines failed and the Republic began taking in water.

Of the 59 passengers and 23 crew on board, all but one were rescued. But the 1,170-tonne ship went down so suddenly that there was no time to rescue the gold coins in the hold. The New York Times of November 3, 1865, reported: “Everything on board, except what the passengers stood in, went down with the ship.”

The 1864 “Liberty Head” coins, the largest gold coin minted by the United States, are valued 138 years later at $4,175 (£2,600) each. If all are recovered, the trove would be worth at least $83.5 million, but the added romance and publicity surrounding the coins could double their value, lifting the total to $180 million.

Video pictures taken by robot submarines almost 300 fathoms beneath the Atlantic clearly show the ship’s deck, paddle wheel and rudder. The surrounding sea is strewn with debris that has been undisturbed since 1865.

The owners of Odyssey Marine Exploration, a company of treasure hunters from Florida that located the wreck after a search team of 20 sonar-scanned 3,000 square kilometres of seabed at a cost of $1 million, are confident they will be able to retrieve the gold.

Greg Stemm, Odyssey’s co-founder, said: “If we wanted to, we could go out with grab buckets and get all the gold in three days”, but he intends a dignified excavation, expected to cost $2 million, because that should enhance the value of the objects recovered.

“It’s enlightened self-interest that compels us to do good scientific work,” he said. “Our actions will speak very loudly to critics who say the private sector can’t do good archaeology.” However, Mr Stemm’s recruitment of 50 more staff for the excavation and a timetable of “days to two months” for recovery of the gold and “another month or two” for the archaeology has alarmed academic marine archaeologists.

“Field work and recovery are the fun part,” said John D. Broadwater, of the National Oceanic and Atmospheric Administration, part of the Commerce Department in Washington.

“It’s the years of analysis and conservation and preparation of a detailed report that usually trip people up. It’s hard to get the commitments of time and funds to do all those things.”

Donald H. Kagin, a coin expert employed by Odyssey, estimates that the $20 gold coins would fetch between $6,000 and $9,000, valuing the total cache at up to $180 million. “That would make it the greatest treasure ever recovered from a shipwreck,” he said.

Mr Stemm said: “Based on legal precedents, we would expect to receive at least 90 per cent of the value of the recovered artefacts.”

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