@RustyCrab @crunklord420 @nobullyplz I think nobullyplz is right, some of the walls seem to be toggeling on and off, allowing us a peak at the void.
That said, most engines have some default fallback, and always clear the color buffer, so that has to be manually overridden, but isn't that complicated using any modern graphics pipeline.

In order to prevent other parts of the level from rendering behind the walls when they vanish, some stencil shader is used, or maybe those parts of the level aren't loaded.

It definitely takes effort though, very cool.