Mainly known due to the Principality of Sealand "wannabe" micronation, the Maunsell Sea Forts are a relic of the Second World War. I wouldn't mind going there for a photographic trip! But in the meantime check out some of the pictures from flickr:
The forts were build during the last world war and housed anti aircraft guns emplacements which protected London and the surrounding sea lanes for incoming raids. In the '60s they hosted a number of pirate radio stations, and eventually one of them ("HM Fort Roughs" just off the Suffolk coast) was occupied and declared an "indipendent nation" (The Principality of Sealand), which apparently will host an Online Casino pretty soon! Nowadays they are rusty and gloomy abandoned structures in the middle of the sea! Up for a boat trip anyone? You can find more information and pictures by following this link.