Island group, French Polynesia. It is the southeasternmost extension of the Tuamotu Archipelago. The largest island, Mangareva, is 5 mi (8 km) long and encircled by a barrier reef 40 mi (64 km) in circumference. Mangareva rises to about 1,444 ft (440 m) in the peaks Duff and Mokoto; the chief village, Rikitea, is on Mangareva's eastern side. The Gambier Islands were annexed by the French in 1881. Their economy is based on subsistence agriculture; pearl harvesting is also of economic importance.