Barry Clifford said his expedition recently located a large metallic mass he believes represents most if not all of the 400,000 coins and other riches believed to have been on the Whydah Gally.

"We think we might be at the end of the rainbow," Mr Clifford said at the recently opened Whydah Pirate Museum on Cape Cod, where many of the expedition's finds are showcased.

Maritime archaeologists and historians say they are intrigued but remain sceptical, mostly because he has been disproved on other finds.

"Barry Clifford's many claims can be very exciting, if they can be verified with photographs or scientific proof," said Paul Johnston, a curator at the Smithsonian's National Museum of American History in Washington DC, who specialises in shipwrecks. "Until then, it's just talk."

The former slave ship, commanded by the Devon-born English pirate Samuel "Black Sam" Bellamy, went down in stormy seas off Wellfleet, Massachusetts, in 1717, killing all but a handful of the nearly 150-person crew.

It is believed the heavily laden ship sunk quickly, leaving the ill-gotten riches from more than 50 ships at the bottom of the ocean.

Victor Mastone, chief archaeologist for the Massachusetts Board of Underwater Archaeological Resources, which oversees shipwrecks and other undersea finds, suggests the pirates could have simply been lying.

"Did they brag more than they should have? Who knows?" he said. "We know what the pirates said they had."

Mr Clifford dismissed Mr Johnston and others as long-time opponents who have refused to treat his team's work seriously.

"Why would they be bragging to the judge about how much treasure they stole? They were hanged," he said, referring to the fate of the surviving pirates of the Whydah.

The 71-year-old explorer hopes to start investigating the suspected riches this month, but stressed the recovery process will take time. Once the mass is located and raised, his team will need to gently break it down using electrolysis and small hand tools.

"For me, it'd be great to get it all finished, but it isn't going to get done in my lifetime," Mr Clifford said. "Archaeology doesn't happen quickly, if you're doing it correctly."

Since his 1984 discovery, Mr Clifford and his team have returned nearly every year to the wreck, over which he has special rights.

They have already reclaimed 200,000 artifacts, including thousands of silver Spanish coins, hundreds of pieces and fragments of rare African gold jewellery, dozens of cannons, various colonial-era objects and other prizes.

A new find at the wreck that made him famous would be a coup for Mr Clifford, who has been dealt major setbacks on other recent expeditions.

In 2014, he claimed to have found the wreck of the Santa Maria, Christopher Columbus's flagship from his first voyage to the Americas in 1492, off the coast of Haiti, only to have researchers from Unesco conclude it was more likely to have been a ship from a later era because of the presence of bronze and copper fasteners.

Last year, Mr Clifford claimed to have located the infamous Scottish pirate Captain William Kidd's Adventure Galley off the coast of Madagascar. Unesco again threw cold water on the pronouncement, concluding an over 100lb silver ingot Mr Clifford produced as proof of his find was 95% lead.

Ulrike Guerin, an underwater heritage specialist at Unesco, declined to comment on Mr Clifford's latest claim but said the Haiti and Madagascar experiences highlight how the explorer's work lacks the "necessary scientific approach".