situated roughly 6 n mile
from the coast of the UK
. Originally formed in 1942 after the start of WWII
as a naval military outpost located in the North Sea
called Roughs Tower (or "The Roughs").
The platform is roughly the size and shape of a full-sized basketball court, raised upon two 8 m x ~16 m (26' x 52') concrete mounts, which sit upon two water-filled pontoons, dropped upon the sand of the UK continental shelf. It included space for 100 officers, galleys, radio equipment, and an operable gun tower.
After WWII, Roughs Tower and numerous other offshore forts like it were abandoned by the military. In the 1960s, many of these abandoned forts were occupied by various music personalities and were used to run (basically) pirate radio stations. In 1965, Roy Bates (aka Prince Roy of Sealand) occupied Knock John Tower and operated a station from it until he was arrested, fined, and the practice made illegal. Afterwards, in 1967, the equipment was moved to Roughs Tower, which Bates then moved to with his wife, claimed as an independent state called Sealand, and named himself prince and reigning monarch.
It happened that, under UK laws and international agreements in place at the time, although most of the offshore military platforms were within UK territorial waters, Roughs Tower / Sealand was determined not to be. As a result, Sealand enjoys tacit "grandfathered" recognition of its independence from UK rule even though subsequent laws/agreements have extended the nautical control rights of nations.
Sealand's government reports that it has defended itself from raiders and UK warships as well as successfully overthrown one foreign occupation and held extradition talks with a German diplomat.
A number of criminal organizations are alleged to have falsely represented themselves as representatives of the government of Sealand, including the owners of the site http://www.principality-sealand.com, undertaking unsanctioned activities ranging from selling false passports to attempting to buy arms from Russia.
The United States as well as nearly all other nations have not made any recognition of Sealand as an independent state. If recognized, Sealand would be the smallest independent state in the world, dwarfed by even the Vatican.
In 1999 the HavenCo company purchased all "land" on Sealand from the princely family for the construction of a internationally unrestricted and secure data haven.
http://www.sealandgov.com/ - official page of the govt. of Sealand
http://www.havenco.com/ - page of the HavenCo company
http://freespace.virgin.net/line.design/forts/radioforts.htm - "Offshore Echoes", chronicling the 1960s pirate radio activities from UK offshore forts