The options for supervillain real estate look shockingly similar to the options for a libertarian state:
I. Unclaimed Land
You may be surprised to learn that there are a (very) few places left on Earth that are unclaimed by any sovereign nation. Perhaps the most reasonable is Bir Tawil, a 770 square mile stretch of desert between the borders of Egypt and Sudan. There isn’t a whole lot there, but at least it’s relatively close to more interesting places, and the neighbors are probably too concerned with their own problems to care about a supervillain moving in next door.
The other major possibility is Marie Byrd Land, which is part of Antarctica. At over 620,000 square miles it’s comparable in size to Mongolia or Iran and would be the 19th largest country in the world if it were one. While no countries lay claim to this land, the Antarctic Treaty of 1959 expressly prohibits “any measure of a military nature, such as the establishment of military bases and fortifications.” Although a supervillain, as a private entity, would not be bound by the treaty, that language might provide the basis for joint military action to oust a supervillain operating out of Antarctica. The treaty does state that “Antarctica…shall not become the scene or object of international discord,” but it is doubtful that such language would give the signatory nations much pause before launching the cruise missiles.
A third possibility is purchasing an island from a sovereign nation, but it may be difficult to convince the owner to give up all claim to the island. Ordinarily private islands like Richard Branson’s Necker Island still remain the sovereign territory of a nation (in that case the British Virgin Islands). But there are many impoverished island nations, and an enterprising supervillain may attempt to strike a Faustian bargain for sovereign territory.
Unfortunately, being stuck on land makes a supervillain an easy target, and unless the supervillain can gain international recognition and thus sovereign status, the base is likely to be attacked without legal repercussions. The main benefits here would be isolation and a lack of direct government oversight, not a legal shield against reprisal.
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II. The High SeasIf no land is available or if mobility is a concern, then a supervillain can consider the oceans. The primary governing treaty is the United Nations Convention on the Law of the Sea. This gives some freedoms, including the right to build artificial constructs, but it also prohibits claims of sovereign territory, so a supervillain probably could not create a new floating nation. Still, as long as he avoided making territorial claims, there doesn’t seem to be any legal reason that a sufficiently large floating construct couldn’t just sail around forever.
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But there are other problems for a seafaring supervillain, most particularly the lack of a national flag (presuming that a supervillain would not long be able to fly even a flag of convenience). Article 110 provides that a warship may board a foreign ship on the high seas if “there is reasonable ground for suspecting that…the ship is without nationality.”
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III. Outer SpaceOuter space probably represents the best bet for a supervillain. Although the supervillain and his or her base would not have much in the way of direct legal protections in space or on the Moon, he or she would be protected indirectly by the Outer Space Treaty. The OST bans placing “in orbit around the earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction, install[ing] such weapons on celestial bodies, or station[ing] such weapons in outer space in any other manner.”
Other options include hiding within a population center, becoming the ruler of a sovereign nation or, as a variant of the high-seas option, building a hidden under-water base.