It's a cinematic moment: after hours of close-fought battles underground, you finally break out into the sunlight of what appears to be Arizona. And the endless industrial architecture gives way to rust-coloured rocks and rusting-pocked metal.

Half-Life is a game set inside a sprawling secret government facility. A friend once described it as "a poem to concrete" (he was only half-joking). The acutely observed variations in industrial architecture are beautifully recreated, using the limited textures of the time.

But the moment when you break free of the corridors would not have been possible, using the unaltered Quake engine, with it's weird, wrap-around clouds.