Once upon a time there was an open source game called "Cube" which was a first person shooter. It allowed in-game multi-player level editing, with all the occlu

Once upon a time there was an open source game called "Cube" which was a first person shooter. It allowed in-game multi-player level editing, with all the occlusion and lighting recomputed on the spot. It was superseded by Sauerbraten, which improved the level structure to allow multiple floors, but didn't run as quickly due to the calculations being a bit harder.