The Cook Islands are split into two groups: the low-lying coral atolls of the northern group, most of which rise from the Manihiki Plateau; and the mainly high volcanic islands in the south. The largest island, Rarotonga, rises 2,140 ft (652 m) above sea level and 14,800 ft (4,500 m) above the ocean floor. The islands were settled in about 300 BC by people originating from eastern Melanesia, who migrated via Fiji, Tonga, and Samoa. Polynesians were expert ocean explorers and perfected the use of many navigation aids at a time when European mariners relied on keeping the land in sight. Their techniques included using the stars; knowledge of currents, winds, and wave patterns; and the flight of birds. Captain James Cook was the first European to sight the Cook Islands in 1773. Russian sailors named the islands in his honor in the 19th century.