English: Knock John Naval Maunsell Fort. Knock John Fort was floated and placed in its current position in August 1942 as a defence against enemy shipping and aircraft during the Second World War. It is the only surviving naval sea fort of its type in the Thames Estuary (though a similar platform is still occupied today and exists as the Independent Principality of Sealand). Soon after the War it was abandoned by the navy only to be briefly occupied between 1965 and 1967 by Radio Essex, a pirate radio station. More recently it's the home of many seagulls. It's further out than Shivering Sands and Red Sands, but can still be seen from the north Kent coast on a really clear day, being described as a "pagoda on the horizon".