“Terra nullius.” Under international law, this Latin phrase meaning “nobody’s land” describes an area of land unclaimed by any sovereign nation-state. In an age where nearly every inch of the world’s landmass has been mapped, examples of terra nullius are exceedingly rare.

Bir Tawil, for instance — a desolate parcel of desert on the Egyptian-Sudanese border. Egypt and Sudan have competing maps of the border, but because of the crisscrossing of those boundaries, Bir Tawil would belong to whichever nation relinquished its claim on a larger, more desirable area. Each country insists that Bir Tawil belongs to the other.

This unusual status has attracted self-styled nationbuilders — so- called “micronationalists” — hoping to create their own “countries.” One notable attempt was by Jeremiah Heaton, an American who in 2014 founded the unrecognized “Kingdom of North Sudan” in Bir Tawil. His motivation, he told CNN’s Don Lemon at the time, was to make his daughter a real-life “princess.”

Other micronationalists have turned to Europe. Along the Croatian-Serbian border, a drawn-out dispute along the River Danube has resulted in claims of terra nullius around at least four small pockets of land.

The boundary dispute began in 1947, after World War II, and flared again in the 1990s when attempts were made to restore the historical separation between Serbia and Croatia, which had spent most of the 20th century as subdivisions of Yugoslavia.

Historically, the two territories were separated by the Danube. Serbia claims its border runs down the center of the contemporary river; Croatia claims a different border, based on 19th-century land ownership maps, when the river ran a different course.

These differing border definitions mean Croatia lays claim to land on the eastern bank of the Danube — territory Serbia considers its own. But the river’s former course means there are several pockets on the western bank that would belong to Serbia under those older maps.

As with Bir Tawil, these western pockets of land would go to whichever country loses the larger territorial disp