Scattered around the planet are tiny zones of land unclaimed by other nations just waiting for someone to claim, and in many cases they have already been claimed. Micronations pop-up when someone claims one of these locations and declares it to be their own nation or kingdom. One of the most notorious examples is just off the coast of England. Sealand is a former ocean fortress built by Britain to defend the country. After the wars it was deserted and laid outside the boundaries of the United Kingdom. An Englishman and his family claimed Sealand and established his own country, which has since been overthrown, retaken, and set afire.
Lonely Planet has released a fascinating book titled Micronations: The Lonely Planet Guide to Home-Made Nations that explores other tiny nations that have popped up all around the planet, even in the midst of the continental USA. You can get a copy of book for just $2.97.
Sealand Ocean Fortress