Horizon: Zero Dawn, which is celebrating its second anniversary today, has officially sold “well over" 10 million copies worldwide.

Via the PlayStation Blog, Guerrilla Games managing director Hermen Hulst revealed the number, saying “The idea that we were able to bring Aloy’s journey to so many players is astounding. We’ve been deeply moved by the enormous amount of fan creations you’ve posted online, as well as the heartfelt letters you’ve sent sharing your favorite moments from Aloy’s journey."

Along with announcing this sales milestone, Hulst took the opportunity to reveal little-known details about the development of the game. He revealed that prototyping gameplay elements began as far back as 2010, when most of the studio was still finishing Killzone 3, and because they didn’t yet have an in-game model for Aloy, they used the model for Jammer, an intruder pilot from Killzone 3, in early testing phases of the game.

It appears they used this model while testing the first machine that the development team designed and finished: the robotic T-Rex Thunderjaw. But in gameplay prototyping, the beast looked more like a child’s toy made from plastic bricks. This was a practical design, as each colored brick represented a different part of the beast that could be shot off.

The toy block Thunderjaw and Jammer model. Credit: Guerrilla Games.

In line with the Jammer prototype, one of the early prototype machines ended up as a holographic Easter egg in Killzone Shadow Fall, a PlayStation 4 launch title. The design didn’t make it into Zero Dawn, but many of its behavioral routines were used to create the Sawtooth, and an image of the hologram can be found here.

Many know that the sounds the machines in Zero Dawn make are composed of various different noises, but Guerrilla specifically noted that the Watcher’s sounds actually contain Chihuahua barking. Furthermore, the team’s senior sound designer Lucas van Tol researched over 750 bird sounds, only 30 of which made it into Zero Dawn. Hulst revealed that each bird sounds differently based on the time of the day, but weather, biome location and hight also impact the sound.

Since Zero Dawn’s release in 2017, various updates have been made to the game, including the major Frozen Wilds DLC. In our review of The Frozen Wilds, we called it “Great” saying "Developer Guerrilla has doubled down on the thrill of the hunt with more challenging machines and testing side quests that are added to the original map through an intriguing new storyline that delves into the mystical side of this post-apocalyptic world."

In July 2018, it was revealed that Guerrilla Games was expanding in order to make games faster.

Colin Stevens is a news writer for IGN, and now he wants to play Zero Dawn again. Follow him on Twitter.