The Marshall Islands consist of 29 coral atolls and five small islands in the western Pacific. The atolls lie on top of ancient volcanic peaks that are thought to have erupted from the ocean floor 50–60 million years ago. They include Kwajalein, the largest atoll in the Pacific at 1,000 square miles (2,500 square km), and Bikini and Enewetak atolls, which were used by the US for testing nuclear weapons between 1946 and 1962. Human pressures on these two remote, evacuated atolls have been minimal during the past 50 years, and marine life around them now thrives; for example, 250 species of coral and up to 1,000 species of fish have been recorded at Bikini.
Most of the seamounts scattered across the floor of the western Pacific are far from any plate boundary. The seamounts are found in groups, often strung out in lines running southeast–northwest—the direction of motion of the Pacific Plate. They are caused by hotspots in Earth’s mantle, which periodically punch through the ocean crust to form volcanoes. Some may reach the surface as islands and in the Marshall Islands, coral atolls were formed as the plate moved away from the hot spot and the volcanic islands subsided. The atolls of Enewetak and Bikini were chosen for their remote location as the site of American nuclear bomb tests in the 1940s and 1950s. Several ships were sunk in these tests, but their wrecks are now considered safe for recreational diving.