The source code for E.X.P.L.O.R.: A New World is now available on GitHub. The main branch is located at https://github.com/beemfx/EXPLOR.
We've taken care to make the source code as easy to work with as possible (we are not saying it 'is' easy, just as easy as possible). We've included demo content made specifically for the source code release that demonstrates the game's capabilities so that anyone is free to work with the code without requiring the retail version of the game.