A Northern Oddity
The northernmost point of the continental United States, excluding Alaska, belongs to the state of Minnesota. But it is not found along the main stretch of the Canada border. It sits in a strange little patch of land called the Northwest Angle, a piece of Minnesota that pokes up above the rest of the country into Canada.
The Only Place North of the Line
For most of its length, the border between the United States and Canada in the west follows a straight line known as the 49th parallel. The Northwest Angle is the single exception. It is the only part of the lower 48 states that lies north of that line. On a map it looks like a small chimney of land sticking up out of Minnesota, almost completely surrounded by Canada and water.
A Mapmakers Mistake
The Northwest Angle exists because of an old mistake. When the border was first agreed after the American Revolution, the people drawing it used a map that was simply wrong about local geography, especially about where a nearby river began. The error left this odd notch of land on the American side. Rather than redraw everything later, both countries kept the quirky border. Today the Angle is so cut off that the only land route to the rest of Minnesota passes through Canada.
Source
This article was written using information from Wikipedia.