Manihiki , atoll, c.2 sq mi (5.2 sq km), South Pacific, in the Cook Islands. It comprises 12 islets; the whole group that includes Manihiki and Penrhyn is also often designated Manihiki. Manihiki was discovered in 1822 by Americans and became a British protectorate in 1889. In 1901 it became part of the New Zealand Cook Islands administration. Copra and pearl shell are the main exports. Manihiki is also known as Humphrey Island.