BOSTON (AP) — Fog was swallowing his ship’s bow, the winds were picking up and undersea explorer Barry Clifford figured he needed to leave within an hour to beat the weather back to port.

It was time enough, he decided, for a final dive of the season over the wreck of the treasure-laden pirate ship, Whydah, off Cape Cod.

That Sept. 1 dive at a spot Clifford had never explored before uncovered proof that a staggering amount of undiscovered riches — as many as 400,000 coins — might be found there.

Instead of packing up for the year, Clifford is planning another trip to the Whydah, the only authenticated pirate ship wreck in U.S. waters.

“I can hardly wait,” he said.

The Whydah was built as a slave ship in 1716 and captured in February 1717 by pirate captain “Black Sam” Bellamy. Just two months later, it sank in a ferocious storm a quarter mile off Wellfleet, Mass., killing Bellamy and all but two of the 145 other men on board and taking down the plunder from 50 vessels Bellamy raided.

Clifford located the Whydah site in 1984 and has since documented 200,000 artifacts, including gold, guns and even the leg of a young boy who took up with the crew. He only recently got indications there may be far more coins than the roughly 12,000 he’s already documented.

Just before his death in April, the Whydah project’s late historian, Ken Kinkor, uncovered a Colonial-era document indicating that in the weeks before the Whydah sank, Bellamy raided two vessels bound for Jamaica. “It is said that in those vessels were 400,000 pieces of 8/8,” it read.

The 8/8 indicates one ounce, the weight of the largest coin made at that time, Clifford said.

“Now we know there’s an additional 400,000 coins out there somewhere,” he said.

The final dive may have provided a big hint at where. Diver Rocco Paccione said he had low expectations when Clifford excavated a pit about 35 feet below the surface and sent him down. But his metal detector immediately came alive with positive, or hot, readings.

“This pit was pretty much hot all the way through,” he said.

The most significant artifact brought up by Paccione was an odd-shape concretion, sort of a rocky mass that forms when chemical reactions with seawater bind metals together.

X-rays this week revealed coin-shaped masses, including some that appear to be stacked as if they were kept in bags, which is how a surviving Whydah pirate testified that the crewmen stored their riches.

Clifford doesn’t sell Whydah treasures and said he would never sell the coins individually because he sees them as historical artifacts, not commodities. But he has given coins away as mementos. Two have been sold at the Daniel Frank Sedwick LLC auction house in Florida, with the highest going for about $11,400. The price per Whydah coin would plummet if tens of thousands hit the market, but a retail price of $1,000 each is a reasonable guess, said Augi Garcia, manager at the auction.

Ed Rodley, who studied Whydah artifacts during graduate studies in archaeology at the University of Massachusetts Boston, said the Whydah site keeps producing treasure decades after its discovery partly because it’s so tough to work.

The site is on the edge of the surf zone, where waves start breaking toward shore. Clifford needs seven anchors to hold the boat in place and the murky ocean bottom is just as active underneath him. Rodley said any pits dug by archaeologists would collapse within hours.

What Clifford has gradually gotten to, three centuries after the Whydah went down, is impressive, Rodley said.

“It’s crazy the stuff that’s come out of that site and keeps coming out of that site, year after year after year,” he said.