The Tuamotu Islands are the longest chain of coral atolls in the world, stretching over 1,200 miles (2,000 km). The Tuamotus were settled by Polynesians by AD 700. Ferdinand Magellan was the first European to chart the group in 1521. The islands became known for their rare black pearls, and these still form a major part of the economy. The Tuamotu Islands are part of French Polynesia and the islands of Mururoa and Fantgataufa were used as sites for about 200 French nuclear weapon tests between 1966 and 1996. The atolls were formed as volcanic islands subsided, leaving behind their fringing coral reefs. The islands rise from the Tuamotu Ridge, a plateau of volcanic material formed about 63–40 million years ago. The Gambier Islands at the southeast end of the ridge are younger, with their volcanic peaks still standing above sea level.