The Maunsell Sea Forts were constructed In the Thames estuary in early WWII to protect the Southeastern coast of England from Luftwaffe raids. They were named after their designer, Guy Maunsell. The technology was later used in the construction of offshore oil rigs. The forts were armored anti-aircraft gun platforms anchored to the sea bed. The forts were decommissioned in 1957, and in the mid 60’s, became home to a series of pirate radio stations until those were outlawed by England in 1967. One of the sites, Rough’s Tower, has been occupied since 1964 and is know as the “Principality of Sealand” which claims independent nation rights as it is located in international waters, although this is unrecognized. 

For more information, look here on Wikipedia. There is a current effort to restore the forts. See Project Redsand for more information.