It is a single-player level for D3 and I personally found it to be the best level I have played for that game. I hope this guy keeps it up becuz it is very very well done. Actually saw critters in the level that I never saw in the game. And he did a fantastic job with the level design. Quite impressed.
Just downloaded it and unzip it into your Doom3\Base folder, start the game and from the console (control, alt, ~) type: map qx/qxd3sp1.http://doom3.filefront.com/file/The_Sigma_Core;31823