The answer is mostly obvious: North Dakota. But there is one tiny, bizarre exception, which I found through Google Earth.
The world is full of amazing and beautiful surprises, and I’m pretty sure that over the years I have spent more time playing with Google Earth than with any actual computer “game.” The browser-based Earth-in-Google-Maps interface is easy to use, but the downloadable Google Earth Pro has clearer images and additional tools like distance measurements and geotagged forum posts.
One day I was looking up the Sanford Underground Research Facility in western South Dakota, and decided to scroll around for a bit. I discovered, to my great suprise, that the borders of Montana and Wyoming don’t quite line up – leaving a less-than-one-mile-long anomaly in the South Dakota border. This means that if you drive north on Albion Road outside of Belle Fourche, you will cross the border into Montana.
And here it is, with the border clearly marked:
At first I thought it was a copyright trap, but Google Earth came to the rescue by showing that someone had taken a photo of a “Welcome to Montana” sign just over the border. Sadly, the photo was hosted on the now-defunct Panoramio site, so it’s gone. But you can still see the shadow of the border sign in the close-up satellite image:
Bonus awesomeness: Driving north on Albion Road also takes you past two derelict nuclear missile silos from the Cold War. And also two other sites that are clearly still in use but completely unlabeled. See if you can find them!
If you’d like to explore for yourself (and you should!), here is the direct link in Google Maps – or download Google Earth Pro, turn on the Borders layer, head northwest from Belle Fourche, South Dakota.
Happy virtual travels!