When you’re standing on top of the skyscraper at the end of Left 4 Dead’s No Mercy campaign you look out over the entire city and see the mountains beyond. These non-interactive backdrops are called skyboxes. The way Valve’s Hammer level editor renders skyboxes is to blow them up from a tiny model version built somewhere hidden in the level. If you noclip in No Mercy and dive down through the skyscraper you can find the skybox model, complete with the dome-shaped textures used to create the moonlit sky. It’s tiny, so you can stomp around like a big grizzled godzilla. What’s more, the moon itself is painted onto the inside of this dome texture. Here’s Bill playing astronaut to demonstrate.

And here's an underside view of the model city. The level is far above.

There will be loads more of these techniques out there, so we're going to keep an eye out and write them up as we find them. Share any you've encountered in the comments.