The Principality of Sealand is a micronation (of the self-declared, unrecognised state-like entity type) that claims as its territory Roughs Tower, a derelict man-made structure located in the North Sea six miles (10 km) off the coast of Essex, UK. It is occupied by the family and associates of Paddy Roy Bates. The population of the facility rarely exceeds five, and its inhabitable area is 550 square metres.
Sealand is undoubtedly the world's best-known micronation, and although its claim to sovereignty and legitimacy are generally not taken seriously, it is nevertheless sometimes cited in debates as an interesting case study of how various principles of international law can be applied to a disputed territory.
Sealand's legitimacy has relied on its ability to repel invading forces (mainly pirates), and the British Army, and a ruling in British Courts that the platform was outside of the jurisdiction of the United Kingdom.
For much more detail, see Wikipedia.