A very early prototype of Left 4 Dead has leaked in a playable state, allowing you to go hands-on with Turtle Rock's original concept.
According to self-described video game information finder Tyler McVicker, this "historic leak" is the first the world (outside of Valve and Turtle Rock, of course) has seen of the Left 4 Dead prototype outside of a small number of screenshots. In a detailed video essay, the YouTuber explains how a map called Zombie City, first developed in Counter-Strike: Source and then Condition Zero back in the early-2000s, eventually became Valve's iconic multiplayer zombie shooter.
It's unclear who's responsible for the leak, but Zombie City is now a playable mod for Counter-Strike: Source that you can download right now. I won't link to the mod because lawyers, but I can confirm that it's out there.
Screenshots of the leaked map reveal a fairly rudimentary city-based map with little in the way of detail, and instead of hordes of zombies, you and up to three friends are left to deal with counter-terrorists who only fight using melee. Likewise, the goal isn't to diffuse any bombs, but rather to simply survive the onslaught of counter-terrorists and make it to the end of the map. That's the original concept that Turtle Rock pitched to Valve co-founder and current president Gabe Newell, and which kickstarted the relationship and eventual acquisition of Turtle Rock by Valve.
As far as video game leaks go, this one's definitely significant, not only because it gives us a peek into the inception of one of the most influential multiplayer first-person shooters ever, but also because we actually get to play the darn thing instead of just look at it.
While Valve has semi-recently confirmed it isn't working on Left 4 Dead 3, that hasn't stopped the rumor mill from churning.