&gt; ...just wondered why the gargantuan effort with a DIY engine.<p>Given that Jonathan Blow is in charge of the project, it&#x27;s almost certainly because existing engines were inadequate for the task.<p>From what I can tell, Rust is the most ambitious Unity project out there. The Witness is <i>substantially</i> more technically ambitious than Rust. Additionally -from what I read on the Rust devblogs-, the Rust guys spend a <i>large</i> amount of time working around Unity&#x27;s limitations, missing features, and parts of Unity that are just buggy. (Granted, the buggy stuff generally <i>eventually</i> gets fixed, but until it gets fixed (if it <i>ever</i> does), you have to work around it.)