New Arabia, the largest single floating structure in the world, first built by Saudi Arabia starting in the year 2004 as a megaproject to match the Burj Khalifa and the massive space stations orbiting in the heavens. It was designed to house 50,000 people and run completely off of renewable energy and upcoming Nuclear Fusion, 15 years later, 2018, construction of the massive ship was coming along quite well, however Saudi Arabia was facing its own internal issues at this time, preceding the collapse and the civil wars that started in the 20's, anyway, the ship had its main reactor, engines and its full floorplan constructed although with no furniture or non-essential fittings installed yet.
A few months down the line when the shortages in 2018 hit Saudi Arabia, many residents from nearby cities boarded the ship without permission, as it was the only thing that still had working electricity and plumbing, many also took items like furnishing with them, the military was reluctant to stop people as the ship was so large and the Saudi's had other things to worry about at the time, and if the ship could help quell rebellion in the area is was a plus.
Over the next year several thousand more people poured into the ship and began inhabiting it, among these people were many of the ships original workers who knew the plans and even some of the ships planners who weren't able to get out of Saudi Arabia. This is how the ship was able to operate even when filled to the brim with unskilled people who didn't know about the inner workings, without these skilled people staying we wouldn't have the nation there is today.
Still in 2019 someone brought up an idea, originally taken as a joke by most others, said idea was "What if we went into international waters and declared independence?" however this out there idea slowly caught on as the structure and people within proved themselves to be self sufficient, using the vertical gardens, solar panels, plumbing systems and everything else installed to operate it without much outside assistance, other than a bit of periodic supply assistance from the UN and nearby countries.
This idea really caught on when the Saudi crisis hit in 2020, Saudi Arabia was facing widespread shortages of electricity, food, water, etc nearly everywhere, several reasons attributed to this with the most major being that Fusion energy had significant developments and price drops, more countries were able to just depend on their own oil supplies instead of purchasing it for what would be a higher price, nearby fighting meant that the ship had to activate its engines for the first time ever and attempt to sail somewhere off the coast, it is an absolute miracle that it didn't sink with the myriad of problems it faced, most of these problems were fixable though.
Then the earlier idea which was taken as a joke back then seemed more and more possible, investors, busniessmen and companies from outside started turning their eyes to the vessel as they thought the vast empty spaces within could be used if the ship became a tax haven, the next few months saw these businesses getting more involved in the ship, funding parts of it and sending material, even though it was only around 30 miles off the coast, and still technically belonged to Saudi Arabia, the former had no control over it at this point.
October 24th, 2020 was when it made its first voyage into international waters, encountered even more issues, which were fixed, and began preparing for a possible declaration of independence and everything that would entail, it was already known that declaring independence was possible, the nearby nation of Autopia had done it all the way back in 2003.
And then on December 27th, 2020, it finally happened, the ship declared its independence as the Republic of New Arabia, something that surprised many was that the Saudi government didn't really care, well maybe its not so surprising as the ship had already operated on its own for 2 years, and was already completely autonomous from Saudi Arabia itself, the country wouldn't have any use for it even if it did try to take it back, and might just create more problems than it already had.
Then started the expansion over the next 20 years, foreign businesses moved in, scientific experiments illegal in other countries were legalized and the country was made a tax haven, attracting tourists and residents from all over the world, it would also establish ties with Gulf Community, later to be formed into United Arab Republic, although it remained independent. A few incidents involving piracy around this time forced New Arabia to build, or purchase a small convoy of warships and escorts that would constantly sail with it, and turn part of its airport deck into a military aircraft storage section and a military base.
Today, New Arabia spends its time slowly sailing around the world with a small convoy of ships, attracting thousands of tourists wherever it goes, even visiting some far off areas such as Antarctica, it also notably helped inspire the creation of seasteads and other floating cities around the world. Saudi Arabia isn't doing so well off, it was never able to transition its economy out of oil and build its own fusion reactors and it still in a civil war to this day, it has made a few diplomatic efforts to reclaim the ship but has never attempted to do so by force.