Definition: An oceanic island, often having a characteristic ring-like shape surrounding a lagoon. Atolls are formed when coral reef grows around a volcanic island that later subsides into the ocean.
Definition: A landform that extends above the surrounding terrain in a limited area. A mountain is generally steeper than a hill, but there is no universally accepted standard definition for the height of a mountain or a hill although a mountain usually has an identifiable summit and a local relief of more than 300m.
