From The Art and Popular Culture Encyclopedia
An artificial island or man-made island is an island or archipelago that has been constructed by people rather than formed by natural means. They are created by expanding existing islets, construction on existing reefs, or amalgamating several natural islets into a bigger island.
Early artificial islands included floating structures in still waters, or wooden or megalithic structures erected in shallow waters (e.g., crannógs and Nan Madol discussed below). In modern times artificial islands are usually formed by land reclamation, but some are formed by the incidental isolation of an existing piece of land during canal construction (e.g. Donauinsel and Dithmarschen), or flooding of valleys resulting in the tops of former knolls getting isolated by water (e.g. Barro Colorado Island).
Artificial islands may vary in size from small islets reclaimed solely to support a single pillar of a building or structure, to those that support entire communities and cities.