Kiritimati Atoll, also called Christmas Atoll, coral island in the Northern Line Islands, part of Kiribati, in the west-central Pacific Ocean. It is the largest island of purely coral formation in the world, having a circumference of about 100 miles (160 km). Kiritimati Atoll was sighted on Christmas Eve in 1777 by the English navigator Captain James Cook. (Kiritimati is the Gilbertese spelling of Christmas.) Although claimed by the United States under the Guano Act of 1856, the atoll was incorporated into the Gilbert and Ellice Islands Colony by Great Britain in 1919. Kiritimati played a role during World War