Johnston Atoll, atoll, 2.8 sq mi (7.25 sq km), central Pacific, c.700 nautical mi (1,300 km) SW of Honolulu, central Pacific, an uninc. territory of the United States. It consists of four islands and reefs. The largest island, Johnston Island, c.3,000 ft (910 m) long and c.600 ft (180 m) wide, has been significantly expanded with dredged fill. The atoll is the site of a wildlife refuge.
Johnston Atoll was discovered by Americans in 1796, or the British in 1807, and was claimed by both the United States and the Kingdom of Hawaii in 1858; it was not, however, included in the state of Hawaii. U.S. companies exploited the island for guano until the 1880s. A bird sanctuary since the 1920s, the strategic island was under U.S. Navy control from 1934 and Air Force control after 1948, now jointly with the Dept. of the Interior.
In the 1950s and 60s the United States conducted a series of nuclear tests there, causing some radioactive contamination of the atoll; the area was also a chemical and biological weapons storage and incineration site. Contaminated soil is now stored in a landfill. Since 2009 it has been part of the Pacific Remote Islands Marine National Monument.