Smartly Dressed Games, developer of the wildly popular free-to-play zombie survival sim Unturned, has released the game's source code for non-commercial use.

"Unturned's source code and project files are now freely available!" the developers wrote in a Steam post. "Whether you have a new feature or total conversion in mind, the sky's the limit."

The source code is available via GitHub. In an accompanying FAQ, Smartly Dressed Games explained that the reason behind the release is to further support the community's creative efforts.

"Unturned's greatest strength is our passionate community," the studio wrote. "Releasing the source files allows you and other players to further expand upon the game as you see fit, building a lasting legacy for the game regardless of the changes we make or how many years pass."

There are some conditions for players and modders who want to use the source code. In the FAQ, Smartly Dressed Games stressed that uses for the source code "must be strictly non-commercial" and that "players must be able to play and access all features and content of your mod for free." Some elements of code have also been removed from the game because Smartly Dressed Games doesn't have the rights to redistribute it, such as code relating to anti-cheat systems and third-party pathfinding programs.

The developers also note that the source code release does not mean that Unturned itself is open source, as the non-commercial license does not meet the Open Source Initiative's definition of the term. It is something that Smartly Dressed Games would like to do in the future. "Once we finish another game and are hopefully able to re-acquire certain rights, we'd like to re-license the U3 SDK under the MIT License."

Unturned was one of the early products of the post-Minecraft/DayZ survival sim craze. Built by Nelson Sexton when he was still a teenager, it was released into Steam early access in 2014, hitting 1.0 three years later.

Nine years on, and although it's long past its peak, Unturned is still going strong, attracting thousands of players every day. In April, Smartly Dressed Games released a huge update that overhauled the game's California map. Developed over six years, the update increased the size of the map sixfold, added over 100 quests and significantly expanded Unturned's endgame.