Undersea explorer Barry Clifford next to a display of silver coins recovered from the wreckage of the pirate ship Whydah, at the Whydah Pirate Museum, in Yarmouth, Massachusetts. Clifford found the sunken vessel off Cape Cod in 1984.

The Portland Science Center is showing an exhibit focusing on artifacts found at the wreck of the Whydah like this English 3-pound cannon recovered from the deck.

In 1998, Clifford holds a cannon ball that came from the wreckage of a fleet of pirate and French warships that sank off Venezuela's coast about three centuries ago.

A flintlock rifle, like those traded for slaves, is on display at the Portland Science Center in "Real Pirates: An Exhibition from National Geographic," which features artifacts from the Whydah.

A grenade with its fuse intact, recovered from the wreck of the Whydah.

The gold treasure of the Whydah.

Brass and pewter buttons found at the Whydah wreckage.

Clifford, right, presents a metal bar he believed to be silver, and part of the treasure of the pirate Captain Kidd, to the president of Madagascar, Hery Rajaonarimampianina, in 2015 on Sainte Marie Island, Madagascar. Tests showed the bar to be lead ballast.