Stephan Savoia / AP Chris Macort, a field archeologist working with the Whydah pirate ship museum, reaches down to check one of the ships cannons at the museum's warehouse in Brewster, Mass.

BREWSTER, Mass. -- He calls it "the yellow brick road" because it's literally sprinkled with gold dust.

This road runs along Cape Cod's shifting seafloor, and undersea explorer Barry Clifford believes it leads to undiscovered treasure from the wreck of the pirate ship Whydah.

About two weeks ago, Clifford and his dive team took a previously unplanned trip back to the wreck site, and Clifford returned more convinced than ever that the road he's exploring is a path to riches.

"We think we're very, very close," Clifford said.

The Whydah sank in a brutal storm in 1717 with plunder from 50 ships on board. Clifford discovered the wreck site in 1984 off Wellfleet and has since pulled up 200,000 artifacts, including gold ornaments, sword handles, even a boy's leg.

But just this year, Clifford learned far more treasure may be resting with the Whydah, the only authenticated pirate ship wreck in U.S. waters.

Colonial-era documents discovered in April indicated the Whydah raided two vessels in the weeks before it sank. Its haul on those raids included 400,000 coins, the records said.

Stephan Savoia / AP A tray holding lead shot, spoons, a metal syringe and the partially crushed barrel of a blunderbuss awaits processing at the Whydah pirate ship museum's warehouse in Brewster, Mass.

A Sept. 1 dive during what was supposed to be Clifford's last trip of the season uncovered evidence he was near those coins. That convinced Clifford he had to make another trip before summer's end. So Clifford and a seven-man crew went back on a three-day trip that ended Sept. 13.

Clifford headed for the "yellow brick road," which refers to a gold and artifact-strewn path extending between two significant sites at the Whydah wreck that are about 700 feet apart — a cannon pile and a large chunk of wood Clifford thinks is the Whydah's stern.

The trove of coins and other treasure likely poured from the stern as the ship broke up and the stern drifted to its rest 300 years ago, he said.

Divers searching the path on the recent trip pulled up several concretions, which are rocky masses that form when metals, such as gold and silver, chemically react to seawater. Diver Jon Matel said one discovery was following another, even though divers were working in "black water," or zero-visibility.

Matel says several feet of a fine seaweed called mung settled in the excavated pits and it was like diving in a vat of black Jello.

"You're going by your feel, your touch, your hands, and the ping of a metal detector," Matel said. "When that thing goes off, it's a great feeling."

X-Rays show all the newly retrieved concretions have coins and gold inside. To Clifford it's more proof of high concentrations of metals and coins being dumped en masse on that spot of sea floor.

Clifford believes two examples that were pulled up on the prior trip are particularly compelling evidence: a cannonball piled with 11 coins and a foot-and-a-half long piece of iron stacked with 50 coins.

"Did all of those coins just happen to fall on this one little piece of iron? Or were there thousands of coins there, and this is just an example of what's left?" he said.

Stephan Savoia / AP Underseas explorer Barry Clifford holds the barrel of a partially crushed blunderbuss he salvaged from the wreck of pirate ship "Whydah" in waters off Cape Cod Massachusetts.

Clifford has no doubt it's the latter, but he'll have to wait until next summer to try to find out.

He's taken 21 trips this summer at a cost of more than $200,000. But the worsening weather and lingering boat problems after a recent lightning strike make another visit impossible until June.

Clifford doesn't sell Whydah artifacts, though he knows the treasure, both uncovered and hidden, has monetary and historic value.

He anticipates the delay until the next trip will be somewhat maddening.

"I'll wake up in the middle of the night this winter and go, 'Oh my God, I know what that means,' when I'm reviewing something from the Whydah," he said. "And then I can hardly wait to get back there in the spring."