Truk Islands, group of islands in the western Pacific, part of the Caroline Islands. All of the major islands and many smaller ones lie within a lagoon almost 40 mi (64 km) wide surrounded by a coral reef. Nearly 40,000 people live on the islands, which were first explored by Europeans in 1825. Possession of the islands passed from Spain to Germany to Japan. After World War II, they came under U.S. control. Today, along with other Caroline Islands, they make up the Federated States of Micronesia.