FATHER JACQUES MARQUETTE AND LOUIS JOLLIET: On this day in 1673, a 35-year-old Jesuit priest and a 27-year-old fur trader began their exploration of Lake Michigan and the Mississippi River, leaving from St. Ignace at the north end of Lake Michigan. From there, they went up the Fox River and then overland (carrying their canoes) to the Wisconsin River, which took them to the Mississippi River. Out of fear of running into the Spanish, they turned back at the Arkansas River. By then, they had confirmed that the Mississippi does indeed run to the Gulf of Mexico.

The route back was different. And this becomes important to the history of the country and especially of the City of Chicago: Friendly Native Americans told them that if they go up the Illinois River and the Des Plaines, rather than the Wisconsin, it would make the trip easier. That’s because the portage distance from the Mississippi watershed and the Great Lakes watershed was shortest there. The Chicago River, which dumped into Lake Michigan was only a short distance away.

If you’ve ever wondered why Chicago grew into a major city so quickly, this is why: Location, location, location.  In the modern world it’s easy to miss how much topographical issues like that mattered (and in different ways continue to matter).  But cities don’t just crop up in random places.