When famed pirates overtook a Portuguese ship in 1721 once laden with treasure (now valued over $138 million), the battle went well for the pirates, who eventually sank the ship off the coast of Madagascar. A pair of American archaeologists now claim they’ve discovered the final site of the lost vessel. Known as the Nossa Senhora do Cabo—translated as Our Lady of the Cape—the ship left India with a Portuguese viceroy and the Archbishop of Goa, 200 slaves, and treasure aplenty. Pirates were ready and Olivier “The Buzzard” Levasseur led an ambush of the ship near Reunion Island in the...