A mass of land completely surrounded by water is generally called an island. Very small islands may be called islets or keys, and a group of geologically related islands is an archipelago.
The two major types of islands are continental and oceanic. Continental islands form on a continental shelf and are geologically related to the nearby continent. Examples are the British Isles and Madagascar. Oceanic islands formed independently, usually through underwater volcanic activity. Examples include the Hawaiian Islands and Iceland. These islands are particularly interesting to zoologists and botanists because, due to their remote locations, they often have unique endemic species that contribute to the study of evolution. A third classification is the man-made island, which are made with various materials, including ocean-floor sand, trash, concrete and earth.