The hook
Twenty minutes from the Gateway Arch stands the largest earthwork in the Americas, at the centre of what was, around AD 1100, a city bigger than London — and almost nobody outside the region has heard of it.
01Why it's worth it
Cahokia is the most significant pre-Columbian site north of Mexico, and the gap between that fact and its public profile is the reason to go.
At its height it covered roughly six square miles and held something like 10,000 to 20,000 people, with a further population spread through the surrounding floodplain. That made it larger than contemporary London. It was a planned city: a Grand Plaza levelled by hand, residential districts, a stockade, and more than a hundred earthen mounds, of which around eighty survive.
Monks Mound is the one you climb. It rises about a hundred feet over fourteen acres, built entirely from earth carried in on people's backs in woven baskets — an estimated 22 million cubic feet of it. From the top you see the plaza below, the other mounds, and, in the distance, the Arch. Two monuments to two very different ideas of what this stretch of river is for, in one line of sight.
Woodhenge, reconstructed to the west, is a circle of red cedar posts aligned to the solstices and equinoxes; the site runs sunrise observances at those dates.
Then the city was abandoned, essentially completely, by around 1350. The reasons are argued over — climate, flooding, deforestation, political fracture — and nobody has settled it. It became a UNESCO World Heritage Site in 1982.
Be honest with yourself about what you will see. These are grass-covered mounds in a flat landscape crossed by a highway, not a ruin with standing architecture. The site does not perform for you. The interpretive centre is what converts it, and going in cold is why some visitors leave underwhelmed. Read something first, or start there.
02How to experience it
Interpretive centre first, always — an hour there makes the mounds legible. Then walk the Grand Plaza and climb Monks Mound. Add Woodhenge, and time an equinox or solstice sunrise if the dates line up. Combine with the Arch: seeing them the same day is the point.
From the locals
The Reddit evidence for Cahokia is thinner than the site deserves, and this dossier will not inflate it. What exists is a St. Louis local answering a trip-planning request and slipping it in among the food and breweries: "if you're into Native American history, check out the Cahokia mounds just across the river in Illinois."
That is one conditional recommendation, so love is scored 1 rather than 2. The depth here comes from what the place is, not from how loudly the internet praises it — which is, in a way, the whole point of the pin. A city that once rivalled London in size sits twenty minutes from a national park, and the most enthusiasm it generates online is a subordinate clause.
It does surface in road-trip planning threads, but note the register — the most-upvoted mention in the whole corpus is a bare statement of fact: "Cahokia Mounds State Historic Site, World Heritage Site, is located in Illinois, just several miles east of the Mississippi River and St. Louis, Missouri." Informative, upvoted, and completely unexcited. That is the pattern for this place, and it is why love stays at 1.