I was as surprised as anyone to discover a Squirtle in my office.

That was the first vicious creature I captured in my Pokémon career, which began last Friday, when several million Americans and I finally got through to the server and downloaded Pokémon Go, which is a thing.

Since then, I have captured six more monsters. I believe I am winning.

In fact, I have discovered a subgame within the game—something that I don’t think the creators of Pokémon Go ever intended for me to find. It started just after that fateful battle with the Squirtle, in CityLab’s offices in Washington, D.C.

Pokémon Go may be the most popular game in a generation, if for only the sheer number of Millennials who have fond memories of Pikachu and crew from their childhoods and lots of disposable time as young adults. But there’s more to the game than nostalgia. The real game—Deep Pokémon Go—is fun and instructive for users of all ages. It’s a Borgesian delight with roots dating back to the Victorian era.

What is a Pokémon?

Pokémons are small monsters that you fight with red-and-white balls. You throw a ball at a squiggly little cartoon animal and the ball captures it, kind of like the containment-unit traps on Ghostbusters. I don’t know why the monsters are fighting you. They don’t fight very well—none has beaten me yet. You also receive eggs that hatch, eventually, maybe? I don’t know. I said I was winning; I didn’t say I’d won.

What is the goal in Pokémon Go?

You must apprehend all of them.

How does Pokémon Go work?

Pokémon Go combines nostalgia for the late 1990s with a handful of games that came before and after, namely Ingress, Geocaching, and Letterboxing.

Nintendo, which owns all these Pokémon things, developed Pokémon Go with Niantic, a former division of Google. Niantic’s first major project was a 2014 mobile game called Ingress, in which “the world around you is not what it seems.”

Ingress was hailed as a breakthrough in location-based gaming. You interact with the in-game world by way of a screen that replicates the map of your location—only peppered with “portals” that allowed you to do stuff. Ingress is also classified as an “exergame,” a portmanteau combining “exercise” and “game.” That means a lot of walking around while messing with your phone.  

Pokémon Go is built on much of the same infrastructure that Niantic used to build Ingress. It also features aspects of Geocaching, another mobile-based game that features fewer Pikachus but a similar location-based game-play model. Geocaching is more like a search for real-life “treasure” hidden by other users than an active competition. In Pokémon Go, one of the goals is to find the very rarest Pokémons or PokéStops, which is a bit like searching for geocached secrets.

What are the rarest Pokémons?

Zortles, Bluurkins, Hoobledies, Merp-Merps, and Stanks. Look, I have no idea. Merp-Merp may be a name I overheard.

Wait, what’s Letterboxing?

Letterboxing is a real-world antecedent to Geocaching, Ingress, and Pokémon Go. It may be the progenitor to all these games. It predates GPS and even smartphones. To get a sense of how old Letterboxing is, check out the main web hub for players, Atlas Quest. That site is more GeoCities than Geocaching.

Letterboxing is a location-based scavenger hunt, of a kind, in which players follow clues to reach geocaches, though they aren’t called that. Letterboxing is a Victorian game widely attributed to an Englishman named James Perrott. In 1854, Perrott built a cairn in Dartmoor in southwest England in which he stashed a glass jar with his calling card. The people who made the trek out to the “lonely, bleak spot” left their own cards in exchange.

The only real difference between Letterboxing and Geocaching is technology. With Geocaching, players use GPS to navigate to geocaches: A physical marker and a guestbook tucked away in the real world. Letterboxing is an analog version that requires players to find and follow physical clues to locate the letterboxes. So it’s harder. It’s also got an Etsy vibe to it: You carry around a personalized stamp and logbook, and when you find a letterbox after following the clues, you leave your stamp in the book inside, and receive the stamp inside the letterbox.

The Letterboxing community is still quite active, although it doesn’t boast quite the same daily users as Pokémon Go, which is already poised to surpass Tinder and Twitter. That’s right: Catching just surpassed swiping as a Millennial fascination.

So what’s the point of Pokémon Go?

Again, the goal in Pokémon Go is to capture completely every Pokémon.

But the larger point of location-based games is to find things in the world that you don’t see every day. And that’s what makes Pokémon Go wild fun: In searching for these weird little critters, you find the things you never knew were on the map.

I’ve walked past James Monroe Park dozens of times in the past year. It’s a stone’s throw from my office. But I never knew it as James Monroe Park and never noticed before the luminous, prism-shaped sculpture standing in the fountain in the middle. That’s Tricorn, a sculpture by Duilio Passariello, and if you check in there on Pokémon Go, you can pick up three little catch-em balls and an egg.

Nor did I notice the color-changing LED lanterns in nearby Edward R. Murrow Park. For that matter, I’d never noticed that park, either. There’s a Poké-check-in and everything.

Now, Pokémon Go didn’t tell me everything there is to know about these locations. In fact, the information is woefully out of date for some spots near my office. Whole cycles of the Foggy Bottom Outdoor Sculpture Biennial have come and gone since the sculptures last appeared that show up on Pokémon Go. And that is where perhaps the greatest potential fun of the game is to be had—writing new Pokéstops on Pokémon Go.

Thanks to its meteoric nostalgia factor, Pokémon Go could become one of the most popular apps in mobile history. Much more than a game, it may be a window on the world, once better functions for editing site data are introduced. Imagine an augmented-reality Wikipedia for the real world around you, with markers for historic buildings, architecture, infrastructure, public art, public history—and for the low, low cost of indulging a Millennial pleasure in fighting Poliwags, Weedles, and Squirtles.

Those features can’t come soon enough: I can’t wait to play a back-end version of the game that involves seeking out, reading about, and editing location data across my neighborhood and city. Pokémon Go promises to deliver the map that covers the territory.

Should I download Pokémon Go?

Of course not! If you don’t wind up downloading a clone on your Android device, then you could end up handing over your entire Google account to Nintendo on your iPhone. Borges will have to wait.