Coral Reef
Tuvalu is island group consisting of nine coral atolls in the South Pacific Ocean, about one-half of the way from Hawaii to Australia. See the link below for more information.
Kiritimati, also known as Christmas Island, is a part of the Republic of Kiribati, which is a small island nation located in the central Pacific Ocean. Kiribati consists of 33 coral atolls and islands, with Kiritimati being one of its largest and most well-known.
Earth has one moon, commonly referred to as "the Moon." It is the largest natural satellite of Earth and the fifth largest moon in the solar system.
A cluster or group of islands is called an "Archipelago"
One of the seven largest land areas on Earth is Antarctica. It is the fifth-largest continent and contains around 9% of the Earth's land area.
Caribbean islands are formed through either volcanic activity, where magma rises from the Earth's mantle to create new land, or through coral reefs building up over underwater volcanic formations, forming atolls and barrier islands.
I know of one. The Great Barrier Reef, located off the shore of Australia. It is supposedly the largest coral reef in the world.
Generally, no. i suppose there are one or two .
The Great Barrier Reef
The U.S. used the pacific atolls because they were isolated from large population centers and because the Marshal Islands (where many of these atolls were) were entirely under U.S. control after WW2, so could freely involuntarily evacuate indigenous populations of the atolls that testing was planned on. The USSR did all of their testing inland for security reasons (they also did not have control over any atolls). The British did much of their testing inland on Australia and some in the pacific on Christmas island. The French did their testing in the Sahara desert. The Chinese did all of their testing inland for security reasons. etc.
The moon is the only natural satellite of the Earth and so is the largest.