In a world that has been explored, GPS'ed and Google Earthed to the point where any random guy in Arkansas can survey any remote Asian wilderness, something we take for granted is that every inhabitable piece of the planet belongs to someone, that at least one group of people lays a claim to every part of the planet where it's remotely feasible to set up a tent and a mailbox. (Antarctic wasteland excepted.)
You would be wrong.
Along the border between Egypt and Sudan, you'll see two regions of dispute, Bir Tawil and the Hala'ib Triangle. In 1899, when the United Kingdom was running things in the area, they set the border at the 22nd parallel. That's the border you're likely familiar with. However, three years later, the British moved the borders around a bit to accommodate some residents who lived closer to and identified more with Khartoum than Cairo. This border moved the Hala'ib Triangle into Sudan and Bir Tawil into Egypt (some Egyptian tribe was using Bir Tawil for grazing).
The Hala'ib Triangle has some decent soil. Egypt has even declared part of Hala'ib as Gebel Elba National Park. Bir Tawil is sandy, rocky, mountainous and generally worthless land, not to mention being much smaller than the Hala'ib Triangle. So, naturally, both countries would prefer Hala'ib. Egypt claims the 1899 borders, which gives themselves Hala'ib and Sudan Bir Tawil, and Sudan claims the 1902 borders, giving Sudan Hala'ib and Egypt Bir Tawil.
Which leaves nobody wanting Bir Tawil. Claiming it would screw up the claim for Hala'ib. A third party would have trouble ruling over it, because they'd have to get through one or both nations to get there.
Not that nobody's tried.