The United Nations is rejecting the assertion of an American underwater explorer who says his search in Madagascar uncovered a silver ingot that belonged to 17th century pirate Captain Kidd.

The U.N. cultural body UNESCO said Tuesday that the so-called silver ingot is actually a piece of ballast that consists almost entirely of lead.

In May, explorer Barry Clifford presented what he said was the silver ingot to Madagascar's president in a ceremony on the island of Sainte Marie, near the country's northeast coast. Clifford's diving team had said the bar was found in a bay off the island.

UNESCO then sent a team to Madagascar to investigate.

William Kidd, known as Captain Kidd, raided ships before being captured and executed in London in 1701.