A "polyp" is any kind of growth that's shaped like a blob growing on a long stalk. It's a gross word. Sea anemones have polyps. So can your sinuses or your colon, and you might want to get those looked at. But there's only one geographic feature on the world map that's called a polyp, and for a century it was in one of the most inconvenient places on earth for cartographic complexity: the border of Northern Ireland.

Ireland and Northern Ireland interlock like jigsaw puzzle pieces.

On the Northern Ireland border, between the U.K. county of Fermanagh and the Irish county of Monaghan, there's an odd irregularity in the border: a big chunk of Ireland surrounded on almost all sides by the hedgerows of Northern Ireland. Locals call it the Drummully Polyp, or Coleman Island, or the Connons. The only thing connecting it to Ireland proper is an isthmus of land about 300 feet wide, which you could stroll across in about a minute. Except for single-point enclaves like this one in Austria, it's the narrowest such border on earth.

Surveyors decided not to attempt surgery on the polyp.

The origins of the border's weird zigzag here are lost in the mists of time. Apparently the medieval McMahon clan won a toehold in the feudal lands of the Maguires, and that border held when the McMahon land became County Monaghan and the Maguire land became County Fermanagh. In 1924, the Boundary Commission toured the polyp, wondering if it should be surgically excised, but in the end it was just simpler to let it be. As a result, the Irish railway line between Clones and the county town of Cavan crossed the border six times in just five miles.

Drummully was a lot of trouble during "The Troubles."

The main highway through Drummully parish can't be reached without crossing into Northern Ireland, which was a huge headache for both sides before the Good Friday peace accords. To guard the border, Ireland had to fly in soldiers from the air, and the military was worried that the I.R.A. might mistakenly target Irish helicopters if they looked too much like British ones. In the northwest of Drummully, a long stretch of the border runs right down the middle of the road, which meant drivers heading south were in Ireland, while drivers heading the other way were on U.K. soil. During a time of great political tension, it made the border almost impossible to police.

Will Brexit make this now-benign polyp a cancer once again?

The Irish border hasn't been guarded for over a decade, so now it's almost impossible to tell when you're navigating the various border zigzags near Drummully. Dublin paints its road markings in yellow, and Belfast in white paint, and that's about the only clue. Oh, and you'll also see billboards encouraging Irish drivers to stop for fireworks, which are still not legal in the South. But if another border buildup comes to pass after Britain leaves the EU, Drummully's Polyp may become a military headache once again.

Explore the world's oddities every week with Ken Jennings, and check out his book Maphead for more geography trivia.