Viewed from above, atolls are either circular, hook-shaped, or long and narrow. Their distinguishing features include an elevation of less than five meters above sea level, limited land area, and their reef detritus and sand composition, which supports only plants tolerant of high temperatures and persistently strong winds such as palms and screwpines. Only the largest of the atolls retain fresh water.
從空中鳥瞰，環礁島或細長如帶、或環狀如圈、或倒鉤如飛鏢，共通點是海拔高度往往不到 5 公尺，陸地面積窄小，由珊瑚石、沙礫構成的土地僅容椰子樹、林投樹等高溫又抗強風的植物生長，且只有在面積較大的環礁島上，才能蓄存淡水。