The owners of the world's "smallest state" - Sealand - have put the quasi-country up for sale.
Access to the platform is by helicopter or boat
The former anti-aircraft platform, seven miles off the Essex coast, was taken over 40 years ago by retired Army major Paddy Roy Bates.
The so-called "independent state" of Sealand is currently home to an internet firm.
The platform, built by Britain during World War II, now has its own flag, passports, currency and stamps.
It was derelict until the 1960s when Mr Bates took over the 10,000 sq ft platform and declared it the independent nation of Sealand.
At the time, the platform was beyond the then three-mile limit of British territorial waters. All this changed in 1987, when the UK extended its territorial waters from three to 12 miles.
Sealand's current "head of state", Mr Bates' son Michael, said he was only 14 when they took over the platform, but now seemed the right time to sell up.
"My father is 85 and my mother in her late 70s and I'm 54," he said. "I believe the project needs a bit of rejuvenation."
"Michael of Sealand" said the family were approached by a Spanish estate agents specialising in selling islands.
The firm, Inmonaranja, has put a price tag of 750m euros (£504m) on Sealand.
However, Michael was reluctant to put a price tag on it.
He said the "micro-nation" included accommodation, offices, a power generator and a chapel.
"What you would normally expect in a small village, really," he said.
During the Bates' time on the platform, they saw off an attempt by the Royal Navy to evict them, and an attempt by a group of German and Dutch businessmen to seize control of the platform by force.
Michael said Sealand had aroused suspicion as well as drama.
"At one time it was regarded by some as the Cuba off the east coast of England, he said.
"People thought we were harbouring missiles or something, and this is despite my father's exemplary military record."
He said the North Sea property complied with international laws.
Michael, who travels to Sealand by helicopter from his Essex base, said he believed Britain was increasingly becoming a "nanny state" and that the sale might attract people wanting to "get away from it all".
The government does not recognise the sovereignty of Sealand.