The Silent Cartographer is Halo's best single player level: the pacing, the scale — at the time — was breath-taking. Now industrious modders have taken that original level, and rebuilt the entire thing from scratch from the ground up.
It looks predictably spectacular.
I say 'predictably' because, guys, the things modders are doing these days are insane. Is it really any surprise that a group of talented people could complete this kind of task? Nah. Does that make it any less awesome? Of course not.
The coolest thing for me is how the team has integrated, not only weapons that didn't even exist in the original Halo, but complete mechanics like vehicle hijacking. Those features didn't make it into the game until the second game, so it's super cool to see it all working in the first game.
This is super impressive. Even the video itself, in terms of its look and production values, is stellar. Well worth a watch. You can read more about the project at this forum thread here. You can download the mod itself here