Coral reef environments have been extensively researched by geologists and biologists. Some of the world's largest oil and gas fields are found in fossilized coral reefs. By studying modern coral reefs, researchers hope to understand the formation of the reefs and the entrapment of petroleum in ancient reefs. Bikini and Eniwetok, two South Pacific atolls (or coral islands), were sites for the testing of nuclear weapons during the late 1940s and early 1950s. Scientists have studied these atolls to determine the effects of the nuclear explosions on rocks and organisms. Coral reefs are important research and teaching tools because they form a natural laboratory in which students can study relationships between organisms and their environments.