An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island in a river or a lake island may be called an eyot or ait, and a small island off the coast may be called a holm. A grouping of geographically or geologically related islands is called an archipelago, e.g. the Philippines. An island may be described as such despite the presence of an artificial land bridge, for example Singapore and its causeway, or the various Dutch delta islands, such as IJsselmonde. Some places may even retain "island" in their names for historical reasons after being connected to a larger landmass by a wide land bridge, such as Coney Island or Coronado Island though these are strictly tied islands. Conversely, when a piece of land is separated from the mainland by a man-made canal, for example the Peloponnese by the Corinth Canal, it is generally not considered an island. There are two main types of islands: continental islands and oceanic islands. There are also artificial islands.