The zombie apocalypse continues to take over the world.

DayZ, the open-world undead survival sim, crossed 2 million in sales today, according to lead developer Dean Hall. This is the standalone, unfinished version of DayZ that debuted on Steam’s Early Access channel on Dec. 17. The $30 PC shooter has consistently appeared in the top-seller list on Valve’s digital-distribution channel since its release. Its success is largely based on its strong word-of-mouth reputation that started when DayZ was just a mod for Arma II: Operation Arrowhead. It is currently No. 3 on Steam’s best-seller list behind only Dark Souls II and a discounted bundle of classic Star Wars games.

DayZ drops players into a desolate city with almost no supplies and whose only residents are zombies and dozens of other real gamers who are all vying for the same resources. This creates interesting situations where players can try to kill one another or work together. And death means you lose everything and have to start from scratch.

“Just over two years ago the DayZ mod released,” Hall wrote on Twitter. “Today, the standalone game just broke 2 million units in under six months.”

Day Z, the mod, first debuted in January 2012 for Arma II. It quickly attracted the attention of gamers outside of the Arma II fan community, and it was especially popular on the gameplay video streaming site Twitch. Hall, a veteran of the New Zealand army, built the mod and then briefly rejoined the military before taking on a contract to develop a standalone version of DayZ with Arma developer Bohemia.

DayZ is still in development at the studio, and it is one of the biggest names in the PC Early Access movement. Gamers can buy DayZ through Steam even though its purchase page on the service “strongly advises” customers not to buy it unless they understand it is a work in progress. Apparently, over two million people are OK with that.

Bohemia and Hall expect to release a 1.0 “finished” version of DayZ later this year.

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