For those of us who need to do research to get this joke, I already did it. They mean Rubicon River (which is no longer in the north, so don't look for it there, it's on the opposite side of the knee).

[–] [email protected] 2 points 5 months ago

For more context, the Rubicon is famous less among geographers and more among historians. Famously, the governor of a province was not allowed to bring an army south of the Rubicon into Italy, so when Julius Caesar marched south with his army, that is the point at which it was impossible for Rome not to go to civil war. The phrase "crossing the Rubicon" is an English-language idiom (I don't know if equivalents exist in other languages, though I wouldn't be surprised if it's common across countries formerly in the Roman Empire) meaning "passing a point of no return".