The queue snakes off into the distance, curling around the side of the famous old building, almost encircling it.
It's as if we're suddenly back in the Colosseum's heyday thousands of years ago, when Romans probably queued for hours to witness fierce gladiatorial battles.
The only action today, however, will be a lot of jostling for photos.
It's a skeleton - Rome's Colosseum; a shell of what it used to be, but it's still incredibly popular with travellers.
It's the stuff of bucket lists, the rare case of an icon deserving of the title.
The ruin has spawned its own mini-industries, from the guys outside dressed as gladiators trying to entice tourists into a photo, to the gelati stands and countless sellers of touristy tack. People mill around the front in their thousands today, taking photos, or just standing, staring at the enormity of it all - the stadium and everyone around it.
Something similar is happening throughout Rome's old city.
It's the height of summer and the place is crawling with tourists, bumbag-toting worker ants scouring the city for world-famous attractions to snap and take home.
They're here to tick boxes, to mark off the places the guidebook has told them to visit. The Colosseum? Tick. Piazza Navona? Tick. The Spanish Steps? Tick. The Trevi Fountain? Tick. The Pantheon? Tick.
And why wouldn't you? Rome is a box-ticker's delight. For those with their sights set on, well, sights, the Italian capital is where it's at. You don't even have to try; you can just wander the streets aimlessly and you'll stumble upon world-famous attractions.
I've done it. I've been a box-ticker here before. I lined up and went into the Colosseum on a previous visit, and I still can't resist the temptation to make a detour past it again this morning, to feel the buzz of seeing it in the flesh.
But today I step past the lines of tourists, say "no" to the gladiators, and make my way towards the Metro station. Today, for me, will be different. No boxes will be ticked. Instead, I'm going to follow an old cliche: when in Rome, do as the Romans do. And Romans don't go to the Colosseum.
Did you know there's a pyramid in this city? A huge, Egyptian-style burial pyramid, 2000 years old. It sits beside a park and a busy street, and it's rarely visited.
Here's a hint: the closest Metro station is called Piramide, out in the suburb of Testaccio. I step out of the station and walk a few blocks and there it is, an incongruous colossus stuck next to the street.
There's no official box to tick seeing Gaius Cestius's burial site, which is probably why there aren't any other tourists here ticking it. Just locals and a traffic jam.
Walk around a corner, across a small clearing in the pyramid's shadow, and you arrive at a graveyard. It's tree-lined; well-kept. One of Rome's few Protestant cemeteries doesn't appear on many to-do lists, but the poet John Keats is buried here in an unnamed grave and Percy Shelley's remains lie nearby.
And even still, this isn't why I've come to Testaccio.
I have come to do as the Romans do, and it only takes a short walk through the back streets around here to realise what plenty of them are doing: shopping. For food. Occasionally expensive, always delicious, fresh Italian food.
Some are wandering around Testaccio's open-air market, squeezing vegetables and yelling jovially at vendors.
Others, meanwhile, stream in their thousands towards a more contemporary gastronomic shopping experience: Eataly, a four-storey department store filled with the absolute finest produce Italy has to offer.
Do you hear that?
Angels are chorusing, and harps are being strummed. Because Eataly is heaven.
Inside, vendors are offering free tastings of their product, everything from San Daniele prosciutto to hunks of parmigiano reggiano and small glasses of artisanal beer.
There are miniature restaurants throughout the four floors - places specialising in pasta, or pizza, or wine, or meat, or oysters.
Eataly is carefully organised, beautifully presented foodie mayhem. Romans rush around with armfuls of produce, stopping to taste a little something on their way to the cured meats section, or manoeuvring a free hand to grab hold of an espresso.
In this religious city, they've all come to worship food, glorious food. This isn't a box for tourists to tick; it's not historically significant or something culturally rich. It's just a place people go to do what they enjoy.
No one pays me any attention when I finally leave, having ingested half of Italy. No vendor offers to sell a fridge magnet; no fake gladiator pleads for a photo. And there's not a queue in sight.
What's been your best experience doing something locals do in a foreign city? Post your comments below.
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