It’s called Bir Tawil. It’s 800 square miles of really lousy-looking desert in the Sahara.
There are two borders between Egypt and Sudan. Egypt likes the one drawn in 1899. Sudan likes the one from 1902.
On this map, Egypt likes the straight horizontal border. Sudan likes the jagged diagonal one with the little loop on the left side. This means both countries claim the green triangle, and neither one wants the little black smudge.
I can see why.
This was in the news today, as a Virginia man has claimed the area as the new Kingdom of North Sudan.
FWIW: I’ve seen that little sliver on maps before, and I never knew what the deal was. It turns out there are some smaller places that are unclaimed too.