With little more than a sword and a steed, you can go anywhere. Do anything. Be anyone.

That's the lesson behind video games like The Witcher, a medieval fantasy series that lets you roam vast landscapes in search of adventure. Players crisscross the simulated countryside, slaying monsters, talking to townspeople and leveling up their characters. The latest entry in the franchise, Wild Hunt, offers a digital playground that adds up to more than 50 square miles of playable terrain, according to some estimates.

The fundamental promise of open-world games lies in freedom, exploration and choice. And the possibilities associated with this promise have only grown in scale and ambition, right alongside the technology supporting it. As consoles, PCs and other gaming devices have become more powerful, developers are poised to make the open-world game the defining genre of this generation.

In recent years, a kind of open-world fever has gripped the gaming industry as it races to create ever-larger and more complex sandboxes for us to play in. Published in 2001, Grand Theft Auto III had a map size of just 3 square miles. Five years later, The Elder Scrolls IV gave players 16 square miles to play around in. That figure ballooned to 104 square miles in 2013 with the release of ArmA 3, a military simulator that imagines a conflict on a fictional Mediterranean island, Altis.

These numbers are pretty abstract, so let's try to put them in perspective. Here's a to-scale rendering of Altis stacked up against other game maps, including the Battlefield franchise, Skyrim, and Red Dead Redemption. It takes 7 hours of real time to walk across Altis, as YouTuber Rapskilion learned (the sped-up footage still takes nearly 10 minutes to get through):

Four of the top 10 best-selling games last year were open-world games: Destiny, Grand Theft Auto V, Minecraft and Watch Dogs.

Although there are older games with truly gigantic maps — here's looking at you, Elder Scrolls II — the terrain and contents of those maps were often randomly generated by the machine you were playing on, making for an inconsistent experience. Or perhaps they were sparsely populated, even if the land stretched on forever. By contrast, the most advanced open-world games today come densely pre-filled with interesting characters, shops, and other things that players can have meaningful, repeated interactions with.

This year's Electronic Entertainment Expo led to a slew of open-world game announcements. Ubisoft introduced what's being called the publisher's largest-ever open-world game in Ghost Recon: Wildlands. It's still working on The Division, which lets players explore a post-apocalyptic New York City. Players can hunt the corrupt across London in Assassin's Creed: Syndicate. Warhorse Studio's Kingdom Come: Deliverance is an open-world medieval simulator. Bethesda shed a little more light on Fallout 4. EA had the goods on Mass Effect: Andromeda. Square Enix showed off Just Cause 3. And that's a non-exhaustive list.

The explosion of sandbox games is reflective of a broader shift in our tastes, but also our attitudes toward the real world. If modern first-person shooters were the defining genre of the last generation of games, the next might be open-world games that subvert what FPSes have become: Rigid, cinematic experiences that offer the player little choice but to literally soldier on through linear levels, prescripted quick time events and a singular storyline.

Open-world games offer a kind of freedom you don't get in Call of Duty. You get to decide how you want your character to grow, what missions you want to undertake, which parts of a city you want to explore. You make the story. Nobody gets to tell you what to do anymore.

If that sounds rather infantile, you may be onto something. Those who grew up with video games perhaps preferred greater direction from their games when they were younger. But now, as that generation matures and becomes acquainted with adulthood — simultaneously bringing gaming into the mainstream — games are becoming more adult, too. They're filled now with more complex choices and issues to match the more complicated universe we actually inhabit in the offline world. In 2015, the average age of a gamer was 35, according to the Electronic Software Association.

And all this makes sense. We know that Millennials — excuse me, snake people — are highly independent-minded, creative people who abhor traditional sources of authority in favor of making their own way. That's consistent with the values of autonomy promoted in games such as The Witcher.

Better technology has a role to play in making all this possible. But we're also in the midst of wider cultural transformation, one that has folks embracing sandbox games like never before.